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Originally appearing in Volume V27, Page 385 of the 1911 Encyclopedia Britannica.
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EMBRYOLOGY 2 AND LIFE-HISTORY We owe to W. E. Castle (1896) the most complete account which has yet been given of the early stages of development in an Ascidian. His careful study of the cell lineage in Ciona has made it clear that some of the conflicting statements of his predecessors arose from incorrect orientation of the embryos. One of the most important of his conclusions is that the mesoderm of Ascidians, and probably that of the archaic Vertebrates, is derived from both primary layers, ectoderm and endoderm. Further, he finds that Ciona produces both ova and spermatozoa at the same time, but self-fertilization very rarely occurs. The eggs are laid just before dawn, ' For structure of other forms, see below. 2 For reproduction by gemmation see under " Classification " below.and the larva is hatched during the following night. The test cells adhering to the young homogeneous test have, it is now well known, no connexion with the cells found later in the adult test. The larvae are free-swimming for from one to several days. They avoid the light. The spermatozoon enters at the ventral hemisphere, and that point determines the median plane and the posterior end of the embryo. The ventral is the animal pole. The cleavage is from the beginning bilateral. The first cleavage plane is vertical, and separates the right and left halves of the embryo. The four smaller dorsal cells with yolk give rise to the endodermal hemisphere; the four larger, more protoplasmic, cells form the ventral ectodermal hemisphere. The cells of the latter hemisphere divide more rapidly, and form the future aboral surface. When the dorsal hemisphere has twenty-two cells the ventral has fifty-four. The gastrulation is a combination of epiboly and invagination. The ventral ectoderm grows over, so as to envelop the dorsal hemisphere, while the latter sinks down and becomes saucer-shaped. In the centre of the dorsal surface ten cells form the future endoderm. Round these comes a ring of cells, the chordamesenchyme ring, from which the notochord and mesenchyme arise. Outside this ring is a row of cells, the neuro-muscular ring. The more anterior of these cells form the medullary plate, the more posterior the longitudinal musculature of the larva. The remainder of the cells (in the 112-cell stage) form ectoderm. By growth at the anterior end the blastopore gets pushed posteriorly, and the anterior chorda cells are covered up, and come to lie in the dorsal wall of the archenteron, sixteen cells in two rows, one over the other. The blastopore closes in the posterior part of the dorsal surface. In front of it is the medullary plate, with a continuation backwards at the sides of the blastopore. This region forms the trunk of the larva, the part posterior to it being drawn out to form the tail. The chorda cells pass back into the tail, while the mesenchyme cells shift forwards into the trunk. The muscle cells, derived from the neuro-muscular ring, lie behind the blastopore, and form the muscles of the tail. The closure of the medullary canal takes place from the blastopore forwards, and then the nerve cord is grown over by ectoderm. After closure of the blastopore the mesenchyme cells lie as lateral masses in the trunk; later they become the blood corpuscles and the mantle cells, &c. Castle also discusses some important theoretical questions. He points out that, in Ciona at least, the chorda-mesenchyme ring takes part along with endoderm in the primary invagination, and so belongs to the primary endoderm; while the rest of. the mesoderm, the muscle cells of the neuro muscular ring, are carried in by a secondary invagination, and belong to the outer layer of the young gastrula, or primary ectoderm. He considers that the chorda must be regarded as a mesodermal organ. He agrees with former observers in seeing no trace of enterocoele formation, and he doubts whether any Chordata are Enterocoela. He does not believe in distinguishing those Metazoa with a mesoderm from those with a " mesenchyme." He considers that embryology gives no support to the Annelid hypothesis as to the origin of Chordates. A long-continued discussion as to the origin, nature and fate of certain cells, the " testa-zellen," which make their appearance between the young embryo and its follicle (fig. 12), has ended in (After Pizon.) practical agreement that these small cells are derived 'tom the follicle-cells, and have nothing to do with the test. In Salpa, how-ever, certain follicle-cells enter the embryo, and perform important functions in guiding the development for a time. In most Ascidians the eggs are fertilized in the peribranchial cavity, and undergo most of their development before leaving the parent; in some cases, however, the eggs are laid, andembryology. fertilization takes place in the surrounding water. The segmentation is complete and regular (fig. 13, A) and results in the formation of a spherical blastula, which then undergoes invagination (fig. 13, B). The embryo elongates, and the blastopore or invagination opening comes to be placed on the dorsal edge near the posterior end (fig. i, C). The hypoblast cells lining the archenteron are columnar in form, while the epiblast cells are more cubical (fig. 13, B, C, D). The dorsal surface of the embryo now becomes flattened and then depressed to form a longitudinal groove, extending forwards from the blastopore to near the front of the body. TI3is " medullary groove " now becomes converted into a closed canal by its side walls growing up, arching over, and coalescing in I converted into lateral muscle bands (fig. 1$, G) and a ventral cord the median dorsal line (fig. 13, D). This union of the laminae dorsales to form the neural canal commences at the posterior end behind the blastopore and gradually extends forwards. Conse- quently the blastopore comes to open into the posterior end of the neural canal (fig. 13, D), while the anterior end of that cavity remains A to F, Longitudinal vertical sections of embryos, all placed with the dorsal surface uppermost and the anterior end at the right. A, Early blastula stage, during segmentation. B, Early gastrula stage. C, Stage after gastrula, showing commencement of notochord. D, Later stage, showing formation of notochord and of neural canal. E, Embryo showing body and tail and completely formed neural canal. F, Larva just hatched; end of tail cut off. G, Transverse section of tail of larva. adp, Adhering papillae of larva. nec, Neurenteric canal. at,' Epiblastic (atrial) involution. oc, Ocular organ of larva. au, Auditory organ of larva. g, Gelatinous investment of ar, Archenteron. embryo. bc, Blastocoele. in, Muscle cells of tail. bp, Blastopore. mes, Mesenteron. ch, Notochord. mc, Mesoderm cells. ep, Epiblast. nv, Cerebral vesicle at anterior hy, Hypoblast. end of neural canal. nc, Neural canal. open to the exterior. In this way the archenteron communicates indirectly with the exterior. The short canal leading from the neural canal to the archenteron is known as the neurenteric canal (fig. 13, D, nec). Previous to this stage some of the hypoblast cells at the front edge of the blastopore and forming part of the dorsal wall of the archenteron (fig. 13, C, ch) have become separated off, and then arranged to form an elongated band, two cells wide, under-lying the posterior half of the neural canal (fig. 13, D, E, ch). This is the origin of the notochord. Outgrowths from the sides of the archenteron give rise to laterally placed masses of cells, which are the origin of the mesoblast. These masses show no trace of metameric segmentation. The cavities (reproductive and renal vesicles) which are formed later in the mesoblast represent the coelom. Consequently the body cavity of the Tunicata is a modified form of enterocoele. The anterior part of the embryo, in front of the notochord, now becomes enlarged to form the trunk, while the posterior part elongates to form the tail (fig. 13, E). In the trunk the anterior part of the archenteron dilates to form the mesenteron, the greater part of which becomes the branchial sac; at the same time the anterior part of the neural canal enlarges to form the cerebral vesicle, and the opening to the exterior at the front end of the canal now closes. In the tail part of the embryo the neural canal remains as a narrow tube, while the remains of the wall of the archenteron—the dorsal part of which becomes the notochord—are of cells, which eventually breaks up to form blood corpuscles. As the tail grows longer, it becomes bent round the trunk of the embryo inside the egg-membrane. About this period the epiblast cells begin to form the test as a cuticular deposit upon their outer surface. The test i's at first devoid of cells and forms a delicate gelatinous investment, but it shortly afterwards becomes cellular by the migration into it of test cells formed by proliferation from the epiblast.' The embryo is hatched about two or three days after fertilization, in the form of a tadpole-like larva, which swims actively through the sea by vibrating its long tail. The anterior end of Larval the body is provided with three adhering papillae (fig. 13, Larva F, adp.) in the form of epiblastic thickenings. In the . free-swimming tailed larva the nervous system, formed from the walls of the neural canal, becomes considerably differentiated. The anterior part of the cerebral vesicle remains thin-walled (fig. 13, F), and two unpaired sense-organs develop from its wall and project into the cavity. These are a dorsally and posteriorly placed optic organ, provided with retina, pigment layer, lens and cornea, and a ventrally placed auditory organ, consisting of a large spherical partially pigmented otolith, attached by delicate hair-like processes to the summit of a hollow crista acoustica (fig. 13, F, au). The posterior part of the cerebral vesicle thickens to form a solid ganglionic mass traversed by a narrow central canal: this becomes the ganglion of the adult Ascidian. The wall of the neural canal behind the cerebral vesicle becomes differentiated into an anterior thicker region, placed in the posterior part of the trunk and having a superficial layer of nerve fibres, and a posterior narrower part which traverses the tail, lying on the dorsal surface of the notochord, and gives off several pairs of nerves to the muscles of the tail. Just in front of the anterior end of the nervous system a dorsal involution of the epiblast breaks through into the upturned anterior end of the mesenteron and thus forms the mouth opening. Along the ventral edge of the mesenteron, which becomes the branchial sac, the endostyle is formed as a narrow groove with thickened side walls. It probably corresponds to the median portion of the thyroid body of Vertebrata. A curved outgrowth from the posterior end of the mesenteron forms the alimentary canal (oesophagus, stomach and intestine), which at first ends blindly. An anus is formed later by the intestine opening into the left of two lateral epiblastic involutions (the atria), which rapidly become larger and fuse dorsally to form the peribranchial cavity. Outgrowths from the wall of the branchial sac meet these epiblastic involutions and fuse with them to give rise to the first formed pair of stigmata, which thus come to open into the peribranchial cavity; and these alone correspond to the gill clefts of Amphioxus and the Vertebrata. A, Ascidia; S, Styela; M, Anurella; C, Compound Ascidian. Fig. 14 shows a few characteristic forms of Ascidian " tadpoles," or free-swimming larvae. A and S are typical simple Ascidians; M is the aberrant tailless form found in some Molgulidae; and C is the larva of a typical compound Ascidian. After a short free-swimming existence the fully developed tailed larva fixes itself by its anterior adhering papillae to some foreign object, and then undergoes a remarkable series of retro- gressive changes, which convert it into the adult petals to Ascidian. The tail atrophies, until nothing is left but Adutt Form. some fatty cells in the posterior part of the trunk. The adhering papillae disappear and are replaced functionally by a growth of the test over neighbouring objects. The nervous system with its sense organs atrophies until it is reduced to the single small ganglion, placed on the dorsal edge of the pharynx, and a slight nerve cord running for some distance posteriorly (van Beneden and Julin). Changes in the shape of the body and a further growth and differentiation of the branchial sac, peribranchial cavity and other organs now produce gradually the structure found in the adult Ascidian. The most important points in connexion with this process of development and metamorphosis are the following: (I) In the ' Some of the first test cells are also probably derived from the epithelium of the egg follicle. (After Bowalevsky.) FI 13.-Stages in the Embryology of a Simple Ascidian. . Ascidian embryo all the more important organs (e.g. notochord, Chordata, inasmuch as it has a longitudinal skeletal axis (the notoneural canal, archenteron) are formed in essentially the same chord) separating a dorsally placed nervous system (the neural manner as they are in Amphioxus and other Chordata. (2) The canal) from a ventral alimentary canal (the archenteron) ; and free-swimming tailed larva possesses the essential characters of the therefore during this period of its life-history the animal belongs to the Chordata. (3) The Chordate larva is more highly organized than the adult Ascidian, and therefore the changes by which the latter is produced from the former may be regarded as a process of degeneration (31). The important conclusion drawn from all this is that the Tunicata are the degenerate descendants of a group of primitive Chordata (see below).
End of Article: EMBRYOLOGY 2 AND

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