246 



CEPHALOPODA. 



to the actual condition of the disc. This is also confirmed by Vialleton's 

 earlier description [e.g., Figs. 25 and 26, No. 48), in which the various condi- 

 tions of the nuclei in symmetrical division are given. Further, whole 

 complexes of cells such as the posterior cells or those of a lateral part may 

 advance more rapidly in their division, whilst the division of others may he 

 retarded, this being again visible in the state of their nuclei. 



In the germ-disc depicted in Fig. 110 two complexes of segments lying 

 symmetrically and marked off by the planes II and III', are distinguished by 

 the fact that the furrows lying between them are less distinct than those in 

 other parts of the germ-disc These segments are thus shown to be connected 

 together, and have most probably proceeded from the segments bounded by 

 the furrows II and III' of the stage represented in Fig. 109 A, which is passed 

 through in Loligo and in Sepia in the same way (Watase). Such a condition 

 also renders the bilateral symmetry specially clear. 



/ 





"' - 5 *-•*■* li 





■ . •• **•'• 



f -r y 



-: 



■ 



i ■ . • ■ •■ ;t *-" -'-* -.-■,- 



- >* > • fc i>-« =< . ' ■ - - ~\ 



-< -*■ ■••.-' f- • e» 'i* ---?■* ^ 



-->^V '• " ■-/ - V 



his / ■ . - • \.Y 



^f,L 





\L. 



Fig. 111. Germ-disc of Sepia officinalis at a late stage of cleavage (after Yialleton). 

 W, blastomeres ; Wc, blastocoues; d, yolk. 



Up to this point, the origin of the cells is clearly recognisable in 

 their arrangement ; their position with regard to the middle line also 

 is very regular. As cleavage proceeds further, and the blastomeres 

 continually decrease in size, this regularity of form is no longer 

 perceptible in the germ-disc. A certain regularity of position may at 

 first be retained in the peripheral cells, especially in the blastocones, 

 but even this is finally lost as the cells continue to increase in 

 number. The animal pole of the egg now appears covered by a 



