EMBRYOLOGY OF CLEPSINE. 267 



250, In regard to the second division of x {d in Robin's figs.) 

 and the destination of the products, Robin's statements and 

 figures are sadly at variance with fact. The cleavage of the 

 dorsal blastomere furnishes the guiding thread to all that 

 follows, and Robin's failure to follow it threw him into a maze 

 of inextricable difficulties. About one hour (8 h. 30) after the 

 formation of the four ectoblasts, the segment x shows a depres- 

 sion which begins a little to the left of the upper and inner 

 angle (fig. 23), and thirty minutes after its first appearance it 

 has completely encircled x (fig. 24, seen from below^). In 15 — 

 20 minutes more this cleavage completes itself cutting o9^ about 

 one-third of the original blastomere as the primary neuroblast 

 (a-i) Several hours now intervene before the second division 

 appears, during which three to five small cells are added to the 

 ectoblasts from the lateral blastomeres {a and i^) . Possibly one 

 or two are added from the neuroblast (.r'), but I am unable to 

 speak with certainty. Figs. 25 and 26 represent a stage reached 

 two hours before stage 27, and correspond with Robin's fig. 



251. Four to five hours after the first division of x (14 h.) it 

 begins to divide again in a direction at right angles with the 

 first plane of division (fig. 27). 



The products of this cleavage are the two mesoblasis {x and xi/^ 

 figs. 28 and 29), one of which [xy) is on a level above with the 

 primary segments a, h, c, fig. 30), and the other occupies the 

 aboral polar field, around which the remaining segments lie in a 

 circle (fig, 29). Robin has represented this central mesoblast in 

 fig. 257. He (tw) supposed that this cell {g) originally 

 occupied a peripheral position and that it had been driven from 

 this position by movements among the cells. " Pendant la 

 duree de ces glissements I'une [g) d'elles est souvent chassee 

 derriere les autres," My figures account for its position without 

 the aid of such movements. According to Robin the entire 

 dorsal blastomere [x) is converted into the " dorsal ectoderm," 

 from which it is evident that he lost sight of the two mesoblasts. 

 The origin of the central mesoblast (.r) is correctly given by 

 Grube (y), but he knew nothing of its character and sub- 

 sequent history. 



{d) Formation of the Neuroblasts. — The neuroblasts are 

 formed by successive divisions of the primary neuroblast {x^) , 

 the first of which begins about 21 hours after the egg is laid 

 (fig. 30), and cuts x^ into two equal parts. Three hours later 

 (24 h.) these two parts [x") occupy the position seen in figs. 

 81 and 32, at which time x and xi/ are still in their original 

 position. The four ectoblasts are now quite surrounded by 

 suialler and less opaque cells which have been produced by the 



