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 connection with the above history is the fate of what have been called the germinal streaks. According to Whitman they are composed of two kinds of cells, viz. four rows of smaller superficial 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 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 Chætopoda, 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 Oligochæta. 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 Gnathobdellidæ. The segmentation of this form has not yet been thoroughly investigated, but Bütschli’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[146]. 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 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 œsophagus 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 Chætopoda. 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 œsophagus 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 præ-oral lobe, is especially prominent. The cilia of the oral region are continued into the lumen of the œsophagus, and at a later period are prolonged, as in Lumbricus, along the median line of the ventral surface.
The mesoblastic bands would seem from Bütschli’s observations, which receive confirmation from Kleinenberg’s researches on Lumbricus, to be prolonged dorsally to the œsophagus 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 Bütschli 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.