266 



CEPHALOPODA. 



prominences; a third and a fourth pair are soon added; this latter pair, i.e., 



the one near the mouth, the most dorsal of the. 

 arms, is always the least developed. The pairs 

 of arms appear very quickly one after the other, 

 so that the above statements are made with a 

 certain reserve. 



In tlm development of Octopus, it is of 

 special interest that the yolk-sac is some- 

 what less developed than in Loligo, as is 

 evident from a comparison of Figs. 122 and 

 123 with Figs. US and 119, p. 261. In 

 earlier stages, this difference is less marked ; 

 later, on the contrary, it is still more notice- 

 able. In itself, this comparatively slight 

 difference in the development of Octopus 

 would be hardly worthy of note, but it 

 affords a transition to those forms in which 



Octopus, seen from the 



funnel - side (original). the yolk-sac develops still less (Argoitauta) 



('/■.arms; mi, optic swell- ".,... , , 



• »_ ". . .... ,,..l..,..l ... ..l +- ..l*-. 4-\ *.:~ ... 



Fig. 1: 



do, 



-Older embryo of 



ings ; inn, nuchal muscle ; 

 rt, retractor muscle of tbe 

 funnel (tr). 



or, indeed, is almost altogether wanting 

 (Grenachek's ( lephalopod). 



(c) Argonauta. 



Argoiututa, in its development, also closely agrees with the forms 

 hitherto considered. Special interest attaches here to the appearance 

 of the shell gland at an early embryonic stage and to its retention 

 for a long period, although the shell of the adult does not develop in 

 it, having another origin and significance, as will be shown later, p. 

 294). The shell-gland, as in Octopus, is said gradually to flatten out 

 again (Kay Lankestek, No. 29 ; Ussow, No. 44, p. 352), and the 

 Argonaut shell forms after embryonic life is over, as was shown by 

 Kolliker in opposition to former statements (Nos. 24, 1 and 9). 



The embryos of Argonauta show, at various stages, in the rise of 

 the different organs, great similarity with the Cephalopods already 

 considered (c/., the late stage depicted in Fig. 124) especially with 

 the embryos of Octopus and Loligo of about the same age (Figs. 123 

 ami 119), but the small size of the yolk-sac seems to determine a 

 more compact form of body. 



As in Loligo, the embryonic rudiment at first extends over a large 

 part of the egg, but becomes at a later stage more concentrated, 

 withdrawing more to the animal pole, and rising from the volk, thus 



