343 



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. 



A 



fJ'S 



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- 



FlG. 159. TWO EMBRYOS OF CLEPSINE IN 

 WHICH THE GERMINAL STREAKS HAVE PARTI- 

 ALLY MET ALONG THE VENTRAL LINE. (After 



Robin ) 



gs. germinal, i.e. mesoblastic streaks. 



The area covered by epiblast is shaded. 

 The so-called neuroblasts at the end of the 

 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. 



