288 



CLEPSINE. 



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 (tig. 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 

 connection with the above history 

 is the fate of what have been 

 called the germinal streaks. Ac- 

 cording to Whitman they are 

 composed of two kinds of cells, 

 viz. four rows of smaller super- 

 ficial cells, which he calls iieuro- 



g-s m 



FIG. 159. Two EMBRYOS OF CLEPSINE IN 



WHICH THE OEBMINAL STBEAKS HAVE PARTIALLY 

 MET ALONG THE VENTRAL LINE. (After Kobin.) 



gs. germinal, i.e. mesoblastic streaks. 



The area covered by epiblast is shaded. 

 The so-called n euro blasts at the end of the 

 germinal streaks are shewn in B. 



blasts, and, in the later stages 



at any rate, a row of deeper large cells, which he calls inesoblasts. 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 xip 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 ner- 

 vous system such as is usually met with in the Chsetopoda, and not a single 

 stage between this and a ganglionated cord is described or figured. Whit- 

 man, 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. 268) as similar to that in Clepsine. In this comparison it 



