The Works of Francis Balfour 2-15
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Foster M. and Sedgwick A. The Works of Francis Balfour Vol. II. A Treatise on Comparative Embryology 1. (1885) MacMillan and Co., London.
- The Ovum and Spermatozoon | The Maturation and Impregnation of the Ovum | The Segmentation of the Ovum | Dicyemae and Orthonectidae Dicyema | Porifera | Coelenterata | Platyhelminthes | Rotifera | Mollusca | Polyzoa | Brachiopoda | Chilopoda | Discophora | Gephyrea | Chaetognatha | Nemathelminthes | Tracheata | Crustacea | Pcecilopoda | Echinodermata | Enteropneusta | Bibliography
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The Works of Francis Balfour Foster M. and Sedgwick A. The Works of Francis Balfour Vol. I. Separate Memoirs (1885) MacMillan and Co., London. Foster M. and Sedgwick A. The Works of Francis Balfour Vol. II. A Treatise on Comparative Embryology 1. (1885) MacMillan and Co., London. Foster M. and Sedgwick A. The Works of Francis Balfour Vol. III. A Treatise on Comparative Embryology 2 (1885) MacMillan and Co., London. Foster M. and Sedgwick A. The Works of Francis Balfour Vol. IV. Plates (1885) MacMillan and Co., London.
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Vol II. A Treatise on Comparative Embryology (1885)
CHAPTER XV. Chatognatha, MYZOSTOMEA AND GASTROTRICHA
THE present chapter deals with three small isolated groups, which only resemble each other in that the systematic position of all of them is equally obscure.
Chatognatha.
The discoveries of Kowalevsky (No. 378) confirmed by Btitschli (No. 376) with reference to the development of Sagitta, though they have not brought us nearer to a knowledge of the systematic position of this remarkable form, are nevertheless of
FIG. 164. THREE STAGES IN THE DEVELOPMENT OF SAGITTA. (A and C after
Hiitschli and B after Kowalevsky.) The three embryos are represented in the same
positions.
A. The gastrula stage.
li. A succeeding stage in which the primitive archenteron is commencing to be divided into three parts, the two lateral of which are destined to form the body cavity.
C. A later stage in which the mouth involution (/;/) has become continuous with the alimentary tract, and the blastopore has become closed.
m. mouth; al. alimentary canal ; ae. archenteron ; bl.p. blastopore; pv. perivisceral cavity; sf>. splanchnopleuric mesoblast; so. somatopleuric mesoblast ; ge. generative organs.
CH^TOGNATHA. 367
great value for the more general problems of embryology. The development commences after the eggs are laid. The segmentation is uniform, and a blastosphere, formed of a single layer of columnar cells, is the product of it. An invagination takes place, the opening of which narrows to a blastopore situated at the pole of the embryo opposite that at which the mouth subsequently appears (fig. 164 A). The simple archenteron soon becomes anteriorly divided into three lobes, which communicate freely with the still single cavity behind (fig. 164 B). The two lateral lobes are destined to form the body cavity, and the median lobe the alimentary tract of the adult. An invagination soon arises at the opposite pole of the embryo to the blastopore and forms the mouth and oesophagus (fig. 164 B and C, m).
At the gastrula stage there is formed a paired mass destined to give rise to the generative organs. It arises as a prominence of six cells, projecting from the hypoblast at the anterior pole of the archenteron, and soon separates itself as a mass, or probably a pair of masses, lying freely in the cavity of the archenteron (fig. 164 A. y ge). When the folding of the primitive cavity takes place the generative rudiment is situated at the hind end of the median lobe of the archenteron in the position represented in fig. 164 C, ge.
An elongation of the posterior end of the embryo now takes place, and the embryo becomes coiled up in the egg, and when eventually hatched sufficiently resembles the adult to be recognisable as a young Sagitta.
Before hatching takes place various important changes become manifest. The blastopore disappears after being carried to the ventral surface. The middle section of the trilobed region of the archenteron becomes separated from the unpaired posterior part, and forms a tube, blind behind, but opening in front by the mouth (fig. 165 A, al). It constitutes the permanent alimentary tract, and is formed of a pharyngeal epiblastic invagination, and a posterior hypoblastic section derived from the primitive archenteron. The anus is apparently not formed till comparatively late. After the isolation of the alimentary tract the remainder of the archenteron is formed of two cavities in front, which open freely into a single cavity behind (fig. 165 A). The whole of it constitutes the body cavity and its walls
3 68
CH/ETOGNATHA.
f/ic mesoblast. The anterior paired part becomes partitioned off into a head section and a trunk section (fig. 165 A and B). The former constitutes a pair of distinct cavities (c.pv) in the head, and the latter two cavities opening freely into the unpaired portion behind. At the junction of the paired cavities with the unpaired cavity are situated the generative organs (ge). The inner wall of each of the paired cavities forms the splanchnopleuric mesoblast, and the outer wall of the whole the somatic mesoblast. The inner walls of the posterior cavities unite above and below the alimentary tract, and form the dorsal and ventral mesenteries, which divide the body cavity into two compartments in the adult. Before the hatching of the embryo takes place this mesentery is continued backwards so as to divide the primitively unpaired caudal part of the body cavity in the same way.
From the somatic mesoblast of the trunk is derived the single layer of longitudinal muscles of Sagitta, and part of the epithelioid lining of the body cavity. The anterior termination of the trunk division of the body cavity is marked in the adult by the mesentery dividing into two laminae, which bend outwards to join the body wall.
The cephalic section of the body cavity seems to atrophy, and its walls to become converted into the complicated system of muscles present in the head of the adult Sagitta.
In the presence of a section of the body cavity in the head the embryo of Sagitta re sembles Lumbricus, Spiders, etc.
The generative rudiment of each side divides into an anterior and a posterior part
In;. [65. Two VIEWS OF A LATE EMBRYO OF
SV.ITTA. A. from the dorsal surface. I?, from the
tide. (After 15iitschli.)
m. mouth ; al. alimentary canal ; v.g. ventral ganglion (thickening of epiblast) ; rp. epiblast ; c.pv, cephalic section of body cavity; so. somatopleure ; s/>. splanchnopleure ; ,;v. generative
CH^ETOGNATHA. 369
(fig. 165, ge]. The former constitutes the ovary, and is situated in front of the septum dividing the tail from the body ; and the latter, in the caudal region of the trunk, forms the testis.
The nervous system originates from the epiblast. There is a ventral thickening (fig. 165 B, v.g) in the anterior region of the trunk, and a dorsal one in the head. The two are at first continuous, and on becoming separated from the epiblast remain united by thin cords.
The ventral ganglion is far more prominent during embryonic life than in the adult. Its position and early prominence in the embryo perhaps indicate that it is the homologue of the ventral cord of Chaetopoda 1 .
BIBLIOGRAPHY.
(376) O. Biitschli. "Zur Entwicklungsgeschichte der Sagitta." Zeitschrift f. wiss. Zoo!., Vol. xxni. 1873.
(377) C. Gegenbaur. " Uber die Entwicklung der Sagitta." Abhand. d. naturforschenden Gesellschaft in Halle, 1857.
(378) A. Kowalevsky. " Embryologische Studien an Wiirmern u. Arthropoden." Mem. Acad. Petersbourg, VII. ser., Tom. XVI., No. 12. 1871.
MYZOSTOMEA.
The development of these peculiar parasites on Crinoids has been investigated by Metschnikoff (No. 380), Semper (No. 381), and Graff (No. 379).
The segmentation is unequal, and would appear to be followed by an epibolic invagination. The outer layer of cells (epiblast) becomes covered with cilia, and the inner is transformed into a non-cellular (?) central yolk mass. At this stage the larva is hatched, and commences to lead a free existence. In the next stage observed by Metschnikoff, the mouth, oesophagus, stomach, and anus had become developed ; and two pairs of feet were present. In both of these feet Chaetopod-like setae were present, which in the hinder pair were simple fine bristles without a terminal hook. The papilliform portion of the foot is at first undeveloped. The feet become successively added, like Chaetopod segments, and the stomach does not become dendriform till the whole complement of feet (5 pairs) are present.
In the primitive covering of cilia, combined with a subsequent indication
1 Langerhans has recently made some important investigations on the nervous system of Sagitta, and identifies the ventral ganglion with the parieto-splanchnic ganglia of Molluscs, while he has found a pair of new ganglia, the development of which is unknown, which he calls the suboesophageal or pedal ganglia. The embryological facts do not appear to be in favour of these interpretations.
B. II. 24
3/0 MYZOSTOMEA.
of segments in the formation of the feet and setae, the larva of the Myzostomea shews an approximation to the Chaetopoda, and the group is probably to be regarded as an early Chactopod type specially modified in connection with its parasitic habits.
BIBLIOGRAPHY.
(379) L.Graff. Das Genus Myzostoma. Leipzig, 1877.
(380) E. Metschnikoff. "Zur Entwicklungsgeschichte d. Myzostomum." Zfit.f. wiss. Zool. y Vol. XVI. 1866.
(381) C. Semper. "Z. Anat. u. Entwick. d. Gat. Myzostomum." Ztit.f. wiss. Zool., Vol. ix. 1858.
GASTROTRICHA.
A few observations of Ludwig on the winter eggs of Ichthydium larus shew that the segmentation is a total and apparently a regular one. It leads to the formation of a solid morula. The embryo has a ventral curvature, and the caudal forks are early formed as cuticular structures. By the time the embryo leaves the egg, it has almost reached the adult state. The ventral cilia arise some little time prior to the hatching.
BIBLIOGRAPHY.
(382) H. Ludwig. " Ueber die Ordnung Gastrotricha Mctschn" Zeit. f. wiss. Zool., Vol. xxvi. 1876.
