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 pahs 

 of arms appear very quickly one after the other, 

 so that the above statements are made with a 

 certain reserve. 



In the development of Octopus, it is of 

 special interest that the yolk-sac is some- 

 what less developed than in Lolif/o, as is 

 evident from a comparison of Figs. 122 and 

 123 with Figs. 118 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 Octopu* 

 would be hardly worthy of note, but it 

 affords a transition to those forms in which 

 the yolk-sac develops still less (Argonauta] 

 or, indeed, is almost altogether wanting 

 (GRENACHER'S Cephalopod). 



(c) Argonauta. 



FIG. 123. Older embryo of 

 Octopus, seen from the 

 funnel - side (original). 

 ar, arms ; au, optic swell- 

 ings ; hm, nuchal muscle ; 

 rt, retractor muscle of the 

 funnel (tr). 



Argonauta, 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 (RAY LANKESTER, 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. 12.'i 

 and 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 yolk, thus 



