DEVELOPMENT WITHOUT ACTUAL YOLK-SAC. 269 



rudiment of the mantle is seen in a later Btage thickly covered with 

 chromatophores. 



It is curious i hat ( <u\:s icher makes no mention of a shell-gland, as an in- 

 ternal shell usually occurs in tin- Oigopsida, and a shell-gland even appears 

 in those Cephalopods which are devoid of the internal shell (Octopus, 

 Argonauta). The region of the animal pole in general, in Grenacheb's 

 Cephalopod, differs from the same region in the other Cephalopods con- 

 sidered by us, and it appears probable that the intermediate stages were 

 missed in consequence of the far from favourable conditions under which the 

 observations were carried out. The mantle is rounded at its end, and the fins 

 which, in Loligo, form so early, appear only at a later Mage. 



We have already, in a Loligo, seen the arms arise Dot far from the 

 vegetative pole (Fig. 115 .4, p. 253). In Gkenacher's embryo, 

 the rudiments of the first two pairs of arms appear as fold-like 

 prominences directly at the edge of the blastoderm which lias not 

 yet completely closed, i.e., quite near the vegetative pole, so that 

 at this stage and still more at a somewhat later stage, when the 

 circumcrescence of the yolk by the blastoderm is completed, the 

 whole yolk with the exception of a small portion is enclosed in the 

 embryonic rudiment. But here also, later, a process takes place 

 similar to that described for some other Cephalopods, /•/:;., the 

 embryonic rudiment withdraws to some extent from the vegetative 

 pole (Fig. 1 26 A) a process which, in the eases before mentioned, 

 led to the formation of a yolk-sac (Figs. 115 and 116, p. 253). 

 Here, indeed, this takes place only to an inconsiderable degree, and 

 thus gives rise to the mere indication of a yolk-sac (Fig. 120 A and 

 B, ds) which, however, becomes somewhat more distinct later (Fig. 

 127, d$). At the time when this swelling, which corresponds to the 

 yolk-sac of other Cephalopods, forms, and in consequence of it, the 

 aperture of the blastoderm, which is still present, undergoes displace- 

 ment, being pressed further away from the vegetative pole towards 

 the so-called nuchal region. This part is for a long time marked by 

 a ciliated area lying somewhat in front of the disappearing aperture 

 (Fig. 12G, w). Anteriorly, the oral aperture (m) appears as an 

 ectodermal invagination. 



When considering the ontogeny of Loligo, a ciliated area lying near the 

 mouth was mentioned (p. 258). The ciliation occurring here in Gbenacheb's 

 Cephalopod in the oral region recalls that area which was compared by 

 Brooks to the velum of other Molluscs. A comparison of the two is, how- 

 ever, inadmissible, since the ciliated area in the present case lies in front of 

 or below the arms, as is evident from Fig. 12(i .1. and still more from Gben- 

 acheb's figures of later stages. The ciliation thus belongs to the region of the 



