268 



CEPHALOPODA. 



Since, in Grenacher's Cephalopod, we apparently have a comparatively 

 primitive form, it has been thought that paucity of yolk might be regarded 

 as the primitive condition. So long as the first ontogenetic processes are not 

 known, no decided opinion can be given on this point, but the development 

 of this form in general shows such close similarity to that of other Cephalopods 

 that a reduction of the yolk seems more probable. We may also regard the 

 early appearance of the chromatophores which elsewhere appear late as 

 secondary (see below). Although the Oigopsida are very primitive forms among 

 the Cephalopods now living {Nautilus and Spirula excepted) they themselves 

 appear highly specialised when fossil Cephalopods are taken into account. 

 There is a great gap between them and the forms with chambered shell, and 

 yet Nautilus even has large eggs rich in yolk, as is shown by examination of. 

 the ovary (Owen, No. 33). [The egg-capsule measures 45 x 16 mm. and the 

 actual egg is 17 mm. long (Willey, No. IV.). — Ed.] 



The earliest development seems to agree with that of the other 

 Cephalopods. A blastoderm forms which, however, in this case very 



soon grows round the yolk. Even 

 before distinct rudiments of organs 

 can be seen on the blastoderm, it 

 has grown almost to the vegetative 

 pole, leaving only a small portion 

 of the yolk free (Fig. 125). The 

 growing edge of the blastoderm is 

 beset with cilia, but these do not 

 lead to any rotation of the embryo. 

 The first indication of organs is 

 the appearance at the animal pole 

 of star-like cells containing red 

 pigment, which rapidly increase 

 in number (Fig. 125. ch). These 

 are the chromatophores which thus 

 form, not as in other ( lephalopods, 

 towards the end of the embryonic- 

 period (Fig. 121), but, strange to 

 say, arise quite at the commencement of that period. They soon 

 spread over the upper third of the blastoderm, at the edge of which 

 a circular fold now forms. The upper pigmented part of the embryo 

 thus rises from the rest of the body as the rudiment of the mantle, 

 this process beginning at the postero-dorsal surface, i.e., in the 

 neighbourhood of the future anal pit, and proceeding towards the 

 antero-dorsal surface. During these differentiations, the originally 

 spherical embryo becomes cylindrical. In Fig. 126 A and 11 the 



b'K 



N& 



a 



125. Young embryo ot Gkena- 

 cher's Cephalopod at the stage In 

 which the blastoderm grows round 

 the yolk (after Grenacher). cA, chro- 

 matophores; d, yolk : r, margin of the 

 blastoderm. 



