The Works of Francis Balfour 2-13
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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 XIII. Discophora
THE eggs of the Discophora, each enclosed in a delicate membrane, are enveloped in a kind of mucous case formed by a secretion of the integument, which hardens into a capsule or cocoon. In each cocoon there are a limited number of eggs surrounded by albumen. The cocoons are attached to waterplants, etc. In Clepsine the embryos leave the cocoon very soon after they get rid of the egg membrane, but in Nephelis they remain within the cocoon for a very much longer period (27 28 days after hatching). The young of Clepsine, after their liberation, attach themselves to the ventral surface of their parent.
Our knowledge of the development of the Discophora is in a very unsatisfactory state ; but sufficient is known to shew that it has very many points in common with that of the Oligochaeta, and that the Discophora are therefore closely related to the Chaetopoda. In Clepsine there is an epibolic gastrula, and mesoblastic bands like those in Euaxes are also formed. In Nephelis however the segmentation is very abnormal, and the formation of the germinal layers cannot easily be reduced to an invaginate gastrula type, though probably it is modified from such a type. Mesoblastic bands similar to those in the Oligochaeta occur in this form also.
The embryology of Clepsine, which will serve as type for the Leeches without jaws (Rhyncobdellidae), has recently been studied by Whitman (No. 365), and that of Nephelis, which will
1 The Discophora are divided into the following groups. I. Rhyncobdellidse. II. Gnathobdellidae. III. Branchiobdellidffi.
DISCOPHORA.
347
serve as type for the Leeches with jaws (Gnathobdellidae), has been studied by Butschli (No. 359). The early history of both types is imperfectly known 1 .
Formation of the layers.
Clepsine. It is necessary to give a full account of the segmentation of Clepsine, as the formation of the germinal layers would be otherwise unintelligible.
Segmentation commences with the division of the ovum into two unequal spheres by a vertical cleavage passing from the animal to the vegetative pole. By a second vertical cleavage the large segment is divided into two unequal parts, and the small one into two equal parts. Of the four segments so produced three are relatively small, and one, placed at the posterior end, is large. Each of the four segments next gives rise to a small cell at the animal pole. These small cells form the commencement of the epiblast, and, according to Whitman, the mouth is eventually placed in their centre. Such a position for the mouth, at the animal pole, is extremely unusual, and the statements on this head require further confirmation.
The posterior large segment now divides into two, one of which is dorsal, and the other and larger ventral. The former I shall call with Whitman the neuroblast, and the latter the mesoblast. The mesoblast very shortly divides again. During the formation of the neuroblast and mesoblast additional epiblastic small cells are added from the three spheres which give rise to three of the primitive epiblast cells, which may now be called the vitelline spheres.
The neuroblast next divides into ten cells, of which the two smaller are soon broken up into epiblastic cells, while the remaining eight arrange themselves in two groups of four each, one group on each side at the posterior border of the epiblastic cap. The two mesoblasts also take up a position on the right and left sides immediately ventral to the four neuroblasts of each side. The neuroblasts and mesoblasts now commence to
FlG. 158. TWO VIEWS OF THE LARVA OF
CLEPSINE. (After Whitman.)
o. oral extremity ; m mouth ; pr. germinal streak.
A. This figure shews the blastoderm (shaded) with a thickened edge formed by the primitive (i.e. mesoblastic) streaks with the four so-called neuroblasts posteriorly. The vitelline spheres are left without shading.
B. represents an embryo in which the blastoderm has enclosed the yolk, and in which the division into segments has taken place. At the hind end are shewn the so-called neuroblasts forming the termination of the germinal streak.
1 Hoffmann's account (No. 36) is so different from that of other observers that I have been unable to make any use of it.
348 CLEPSINE.
proliferate at their anterior border, and produce on each side a thickened band of cells underneath the edge of the cap of epiblast cells. Each of these bands is formed of a superficial quadruple 1 row of neuroblasts budded off from the four primary neuroblasts, and a deeper row of mesoblasts. The compound streaks so formed may be called the germinal streaks.
The general appearance of the embryo as seen from the dorsal surface, after the appearance of the two germinal streaks, may be gathered from fig. 158 A. The epiblastic cap in this figure is shaded. The epiblastic cap, accompanied by the germinal streaks, now rapidly extends and encloses the three vitelline spheres by a process equivalent to that of an ordinary epibolic gastrula; but the front and hind ends of the streaks remain practically stationary. Owing to this mode of growth the edges of the epiblastic cap and the germinal streaks meet in a linear fashion along the ventral surface of the embryo (fig. 159, A and B). The germinal streaks first meet anteriorly (B) and their junction is then gradually continued backwards. The process is completed at about the time of hatching.
During the above changes the nuclei of the vitelline spheres pass to the surface and rapidly divide. Eventually, together with part of the protoplasm of the vitelline spheres, they appear to give rise to a layer of hypoblastic cells. This layer encloses the remains of the vitelline spheres, which become the yolk.
At the front end of the germinal streaks, in a position corresponding with that of the four original epiblast cells, two depressions appear which coalesce to form the single oral invagination ; in the centre of which are formed the mouth and pharynx by a second epiblastic invagination.
The most important point in FIG. 159. Two EMBRYOS OF CLEPSINE IN
connection with the above history WHICH THE GERMINAL STREAKS HAVE PARTIis the fate of what have been ALLY^MET ALONG THE VENTRAL LINE. (After
called the germinal streaks. Ac- ^ germinal, i.e. mesoblastic streaks,
cording to Whitman they are The area covered by epiblast is shaded .
composed of two kinds of cells, The so-called neuroblasts at the end of the viz. four rows of smaller super- germinal streaks are shewn in B. ficial cells, which he calls neuroblasts, and, in the later stages at any rate, a row of deeper large cells, which he calls mesoblasts. As to the eventual fate of these cells he states that the neuroblasts uniting together in the median line form the rudiment of the ventral ganglionic chain, while the mesoblasts equally coalesce and give rise to the mesoblast. Such a mode of origin for a ventral ganglionic chain is, so far as I know, without a parallel in the whole animal kingdom ; and whatever evidence Whitman may have that the cells
1 According to Robin it is more usual for there to be only a triple row of primary neuroblasts.
DISCOPHORA. 349
in question really do give rise to the nervous system he has not thought fit to produce it in his paper. He figures a section with the eight neuroblastic cells in the middle ventral line, and in the next stage described the nervous system is divided up into ganglia ! The first stage, in which the so-called nervous system has the form of a single row of eight cells, is quite unlike any rudiment of the nervous system such as is usually met with in the Chaetopoda, and not a single stage between this and a ganglionated cord is described or figured. Whitman, whose views seem to have been influenced by a peculiar, and in my opinion erroneous, theory of Rauber's about the relation of the neural groove of Vertebrata to the blastopore, does not seem to be aware that his determination of the fate of his neuroblasts requires any special support.
He quotes the formation of these parts in Euaxes (vide preceding Chapter, p. 324) as similar to that in Clepsine. In this comparison it appears to me probable that he may be quite correct, but the result of the comparison would be to shew that the neuroblasts and mesoblasts composed together a mesoblastic band similar to that of the Oligochaeta. Till more evidence is brought forward by Whitman or some other observer in support of the view that the so-called neuroblasts have any share in forming the nervous system, they must in my opinion be regarded as probably forming, in conjunction with the mesoblasts, two simple mesoblastic bands. Kowalevsky has moreover briefly stated that he has satisfied himself that the nervous system in Clepsine originates from the epiblast a statement which certainly could not be brought into harmony with Whitman's account.
Nephelis. Nephelis will form my type of the Gnathobdellidae. The segmentation of this form has not yet been thoroughly investigated, but Biitschli's (No. 359) observations are probably the most trustworthy.
The ovum first divides into two, and then into four segments of which two are slightly smaller than the others. Four small cells which form the commencement of the epiblast are now formed. Three of them are derived by budding from the two larger and one of the smaller of the four cells, and the fourth from a subsequent division of one of the larger cells 1 . The three cells which assisted in the formation of the epiblast cells again give rise each to a small cell ; and the small cells so formed constitute a layer underneath the epiblast which is the commencement of the hypoblast, while the cells from which they originated form the vitelline spheres. Shortly after the formation of the hypoblast, the large sphere which has hitherto been quiescent divides into two, one of which then gives rise in succession to two small epiblastic elements.
The two large spheres, resulting from the division of the originally quiescent sphere, next divide again on the opposite side of the embryo, and form a layer of epiblast there ; so that there is now on one side of the embryo (the ventral according to Robin) a layer of epiblast formed
1 Doubts have been cast by Whitman on the above account of the origin of the four epiblast cells.
350 CLEPSINE.
of six cells, and on the opposite side a layer formed of four cells. The two layers meet at the front border of the embryo and between them are placed the three large vitelline spheres. The two patches of epiblast cells now rapidly increase, and gradually spread over the three large vitelline spheres. Except where they meet -each other at the front edge they leave uncovered a large part of the margin of the vitelline spheres.
While these changes have been taking place on the exterior, the hypoblast cells have increased in number (additional cells being probably derived from the three large vitelline spheres) and fill up in a column-like fashion a space which is bounded behind by the three vitelline spheres, and in front by the epiblast of the anterior end of the embryo. At the sides of the hypoblast the mesoblast has become established, probably as two lateral bands. The origin of the cells forming it has not yet been determined. The hypoblast cells in the succeeding stage arrange themselves round a central archenteric cavity, and at the same time rapidly increase in size and become filled with a secondary deposit of food-yolk. Shortly afterwards a mouth and thick-walled oesophagus are formed, probably from an epiblastic invagination. The mesoblast now forms two curved lateral bands at the two sides of the body, equivalent to the mesoblastic bands of the Chaetopoda. The three vitelline spheres, still largely uncovered by the epiblast, lie at the posterior end of the body. The embryo grows rapidly, especially anteriorly, and the three vitelline spheres become covered by a layer of flattened epiblast cells. Around the oesophagus a cavity traversed by muscular fibres is established. Elsewhere there is no trace of such a cavity. The cephalic region becomes ciliated, and the dorsal part of it, which represents a rudimentary prae-oral lobe, is especially prominent. The cilia of the oral region are continued into the lumen of the oesophagus, and at a later period are prolonged, as in Lumbricus, along the median line of the ventral surface.
The mesoblastic bands would seem from Biitschli's observations, which receive confirmation from Kleinenberg's researches on Lumbricus, to be prolonged dorsally to the oesophagus into the cephalic region. Posteriorly they abut on the large vitelline spheres, which were supposed by Kowalevsky to give origin to them, and to play the same part as the large mesoblasts in Lumbricus. It has already been shewn that the function of the large cells in Lumbricus has been exaggerated, and Biitschli denies to them in Nephelis any share in the production of the mesoblast. It seems in fact probable that they are homologous with the three vitelline spheres of Clepsine ; and that their primitive function is to give origin to the hypoblast. They are visible for a long time at the hind end of the embryo, but eventually break up into smaller cells, the fate of which is unknown.
The embryo of Hirudo would appear from the researches of Robin to develop in nearly the same way as that of Nephelis. The anterior part is not however ciliated. The three large posterior cells disappear relatively early.
DISCOPHORA. 351
General history of the larva.
The larva of Clepsine, at the time when the mesoblastic bands have met along the ventral line, is represented in fig. 158 B. It is seen to be already segmented, the process having proceeded pari passu with the ventral coalescence of the mesoblastic bands. The segments are formed from before backwards as in Chaetopoda. The dorsal surface is flat and short, and the ventral very convex. The embryo about this time leaves its capsule, and attaches itself to its parent. It rapidly elongates, and the dorsal surface, growing more rapidly than the ventral, becomes at last the more convex. Eventually thirty-three postoral segments become formed ; of which the eight last coalesce to form the posterior sucker.
The general development of the body of Nephelis and Hirudo is nearly the same as that of Clepsine. The embryo passes from a spherical to an oval, and then to a vermiform shape. For full details the reader is referred to Robin's memoir.
The presence of a well-marked protuberance above the oesophagus, which forms the rudiment of a prae-oral lobe, has already been mentioned as characteristic of the embryo of Nephelis ; no such structure is found in Clepsine.
History of the germinal layers and development of organs.
The epiblast. The epiblast is formed of a single layer of cells and early develops a delicate cuticle which is clearly formed quite independently of the egg membrane. It becomes raised into a series of transverse rings which bear no relation to the true somites of the mesoblast.
The nervous system. The nervous system is probably derived from the epiblast, but its origin still requires further investigation. The ventral cord breaks up into a series of ganglia, which at first correspond exactly with the somites of the mesoblast. Of these, four or perhaps three eventually coalesce to form the sub-cesophageal ganglion, and seven or eight become united in the posterior sucker.
It would appear from Biitschli's statements that the supra
352 NEPHELIS.
cesophageal ganglion arises, as in Oligochaeta, independently of the ventral cord.
Mesoblast. It has already been indicated that the mesoblast probably takes its origin both in Nephelis and Clepsine from the two mesoblastic bands which unite in the median ventral line. The further history of these bands is only imperfectly known. They become segmented from before backwards. The somites formed by the segmentation gradually grow upwards and meet in the dorsal line. Septa are formed between the somites probably in the same way as in the Oligochaeta.
In Clepsine the mesoblastic bands are stated by Kowalevsky to become split into somatic and splanchnic layers, between which are placed the so-called lateral sinuses. These sinuses form, according to Whitman, a single continuous tube investing the alimentary tract ; a tube which differs therefore to a very small extent from the normal body cavity of the Chaetopoda. The somatic layer of mesoblast no doubt gives rise to the circular and longitudinal muscular layers of the embryo. The former is stated to appear the earliest, while the latter, as in the Oligochaeta, first takes its origin on the ventral side.
A delicate musculature, formed mainly of transverse but also of longitudinal fibres, would appear to be developed independently of the mesoblastic bands in Nephelis and Hirudo (Rathke, Leuckart, Robin, and Biitschli). It develops apparently from certain stellate cells which are found between the walls of the alimentary tract and the skin, and which probably correspond to the system of contractile fibres which pass from the body wall to the alimentary tract through the segmentation cavity in the larva of Chaetopoda, various Vermes and Mollusca 1 .
The mesoblast, so far as is known, gives rise, in addition to the parts already mentioned, to the excretory organs, generative organs, vascular system, etc.
Excretory organs. There are found in the embryo of Nephelis and Hirudo certain remarkable provisional excretory organs the origin and history of which is not yet fully made out. In Nephelis they appear as one (according to Robin, No. 364), or (according to Biitschli, No. 359) as two successive pairs of
1 According to Robin this system of muscles becomes gradually strengthened and converted into the permanent system. Rathke on the other hand states that it is provisional, and that it is replaced by the muscles developed from the mesoblastic somites. It is possible to suppose that it may really become incorporated in the latter system.
DISCOPHORA. 353
convoluted tubes on the dorsal side of the embryo, which are stated by the latter author to develop from the scattered mesoblast cells underneath the skin. At their fullest development they extend, according to Robin, from close to the head to near the ventral sucker. Each of them is U-shaped, with the open end forwards, each limb of the U being formed by two tubes united in front. No external opening has been clearly made out. Semper believed that the tubes were continuous with the three posterior vitelline cells, but this has been shewn not to be the case. Fiirbringer 1 is inclined from his own researches to believe that they open laterally. They contain a clear fluid.
In Hirudo, Leuckart (No. 362) has described three similar pairs of organs the structure of which he has fully elucidated. They are situated in the posterior part of the body, and each of them commences with an enlargement from which a convoluted tube is continued for some distance backwards ; it then turns forwards again and afterwards bends upon itself to open to the exterior. The anterior part is broken up into a kind of labyrinthic network.
The true segmental organs are found in a certain number of the segments and are stated (Whitman) to develop from groups of mesoblast cells. Their origin requires however further investigation.
A double row of colossal cells on each side of the body has been described in Clepsine by Whitman as derived from the mesoblastic plates. These cells (fig. 58 B), which he calls segment-cells, lie opposite the walls of the septa. The inner row is stated to be connected with the segmental organs. Their eventual history is unknown, but they are conjectured by Whitman to be the mother cells of the testes.
The alimentary tract. This is formed primitively of two parts the epiblastic stomodaeum forming mouth, pharynx, and oesophagus, and the hypoblastic mesenteron. The anus is formed very late as a simple perforation immediately dorsal to the posterior sucker.
In Clepsine, where there is an epibolic gastrula, the rudiment
1 Morphologisches Jahrbuch, Vol. iv. p. 676. He further speaks of the tube as " feinverzweigt u. netzformig verastelt," but whether from his own observations is not clear.
B. II. 23
354 DEVELOPMENT OF ORGANS.
of the mesenteron is at first formed of the three vitelline spheres, from the surface of which a true hypoblastic layer enclosing a central yolk mass becomes differentiated, as already described. The mesenteric sack so formed is constricted by the growth of the mesoblastic septa into a series of lobes, while the posterior part forms a narrow and at first very short tube opening by the anus.
The lobed region forms the sacculated stomach of the adult. The sacculations of the stomach by their mode of origin necessarily correspond with the segments. In the adult however the anterior lobe is really double and has two divisions for the two segments it fills, while the posterior lobe, which, as is well known, extends backwards parallel with the rectum, is composed of five segmental sacculations. In connection with the stomodaeum a protrusible pharynx is developed.
In Hirudo and Nephelis the mesenteron has from the first a sack-like form. The cells which compose the sack give rise to a secondary deposit of food-yolk. The further changes are practically the same as in Clepsine. In Hirudo the posterior sacculation of the stomach is primitively unpaired. The jaws are formed at about the same time as the eyes as protuberances on the wall of the oral cavity.
BIBLIOGRAPHY.
(359) O. Biitschli. " Entwicklungsgeschichtliche Beitrage (Nephelis)." Zeit. f. wiss. Zool. Vol. xxix. 1877.
(360) E. Grube. Untersuchnngen iib. d. Entwicklung d. Aniiclidcu. Konigsl)crg, 1844.
(361) C.K.Hoffmann. " Zur Entwicklungsgeschichte d. Clepsineen." Niederldnd. Archiv f. Zool. Vol. iv. 1877.
(362) R. Leuckart. Die mcnschlichen Parasiten (Hirudo), Vol. i. |>. 686, et seq.
(363) II. Rathke. Beit. z. Entwicklungsgesch. d. Hirudineen. Leipzig, 1862.
(364) Ch. Robin. Mfm. sur le Dhjeloppcment embryogenique des Hirudwccs. 1'aris, 1875.
(365) C. O. Whitman. " Embryology of Clepsine." Quart. J. of Micro. Science, Vol. xvm. 1878.
[Vide also C. Semper (No. 355) and Kowalevsky (No. 342) for isolated observations.]
