244 CEPHALOPODA. 



which observers are agreed, the germ-disc becomes markedly bilateral, 

 a character which is retained in the later stages also (Figs. 108 and 

 109 B and C, 110), the two segments mentioned above and the 

 blastomeres that proceed from them retaining their characteristic 

 shape, i.e., retaining their regular position with regard to the median 

 plane, the other cleavage-cells also continuing to be symmetrically 

 arranged with regard to that plane. 



The broad segments of the eight-celled stage (Figs. 109 ^4 and 

 108 B) are directed forward, the narrow segments, on the contrary, 

 backward (Watase, Ussow). In this way the bilateral symmetry 

 of the germ-disc after cleavage and the relation to the form of the 

 adult animal are shown still more distinctly than in the egg before 

 cleavage (rf. p. 239 and Fig. 106 A and B). 



It should be added that Vialleton also describes the striking bilateral 

 symmetry of the germ-disc after cleavage which is evident from Fig. 109, but 

 does not assume so definite a symmetry in the shape of the egg before cleavage. 

 It may be more difficult to establish these points with certainty in the spheri- 

 cal egg of Sepia. The identity of the median sagittal line of the blastoderm 

 with that of the embryo which is emphasised by Ussow is also apparently 

 assumed by Vialleton as probable, so that, according to him also, the 

 bilateral symmetry of the germ-disc after cleavage corresponds to that of 

 the embryo. 



Siuce, in consequence of the continuous division of the cells, the shape of 

 the germ-disc is less regular in the later stages (Fig. Ill), it is very difficult 

 to prove that the bilateral symmetry of the germ disc passes directly over 

 into that of the embryo ; this has, indeed, not yet been exactly proved, so 

 far as we can see. But the bilateral character of the germ-disc found in 

 Cephalopods otherwise very different from one another [Loligo, Sepia, Argon- 

 auta *) makes its relation to the form of the embryo appear more than 

 probable, and we must therefore for the present hold to the view of Watase, 

 although, indeed, this seems to be somewhat conjectural. 



As cleavage advances further, an equatorial furrow cuts off from the 

 two narrow posterior segments, towards the middle of the germ-disc, 

 two small blastomeres (Fig. 109 B) ; a continuation of this equatorial 

 furrow cuts off, in Loligo, similar blastomeres from the large segments 

 in front (Fig. 109 G). In Sepia, however, additional meridional 

 furrows appear first and divide these segments into narrower sections 

 (Fig. 109 B,.IV and V), after which they become divided by an 



* With regard to Argonauta, we have to rely on the statement repeatedly 

 made by Ussow that he saw cleavage taking place, in the forms observed by 

 him, in a similar manner to the above. A confirmation of these statements 

 with respect to the Octopoda is, indeed, very desirable. With reference to the 

 conjectural Sepiola, also investigated by Ussow, see the remarks made p. 236. 



