326 THE CEPHALOPODA 



III. EMBRYOLOGY. 



Our knowledge of the embryology of Cephalopoda is confined to 

 the Dibranchia, the development of Nautilus being unfortunately 

 still unknown. The ovum is remarkable, even in the cases of 

 Nautilus and the ovarian ovum of Spirula (Fig. 285), for the 

 enormous quantity of yolk contained in it. In contrast to all 

 other Mollusca, the segmentation is incomplete : at no period does 

 the ectoderm completely cover in the vitelline mass, so that there 

 is no proper blastopore, or rather the blastopore is enormous and 

 is represented by that part of the vitellus that is not covered by 

 ectoderm (Fig. 290, (2), (3), e). This peculiarity in the development 

 of Dibranchia, however, is only an exaggeration of the phenomena 

 observable in the epibolic ova, provided with an abundant yolk, of 

 certain Gastropoda (Fig. 10, B), and it has been shown that in the 

 archaic Dibranchia (the Oigopsid Cephalopod of Grenacher, Fig. 

 119, D, w) the quantity of yolk is less than 

 in the other members of the order, and that 

 the ectoderm extends much farther over it. 



As the formative protoplasm is localised 

 at the narrower end of the egg, the segment- 

 ation is restricted to this end (Fig. 289, W), 

 and results in the formation of a germinal 

 disc or embryonic area. In the course of 

 subsequent development the embryo is like- 

 wise restricted to this end, and never covers 

 the whole surface of the vitelline mass, on 

 Plo 289. which it appears to be seated (Fig. 291). 



Egg of Loiigo in the first The extent of the embryonic area and of 

 segmentation stage, u, the the free surface of the yolk are in inverse 



four first blastomeres ; m, J 



viteiius. (After Watase.) ratio to one another : the external vitelline 

 mass is smaller in Loiigo than in Sepia, 



smaller still in Argonauta, and reduced to a minimum in the 

 Oigopsida (Fig. 119, D). 



The embryonic area forms the ectoderm : the so-called peri- 

 vitelline or yolk membrane is formed as a proliferation of cells 

 from a limited part of the periphery of the ectoderm, the region 

 of proliferation marking the anal side. The cells thus formed 

 migrate over the whole surface of the yolk and form a layer of 

 scattered nuclei investing it (Fig. 290, (7), h). At a later period 

 the same anal edge of the periphery of the embryonic area gives 

 rise to a second cellular layer, the endoderm : it is at first crescentic 

 in shape, but subsequently becomes ring-shaped, and eventually 

 forms a continuous circular sheet below the ectoderm (Teichmann). 

 At a still later period the ectoderm gives rise to cells constituting 

 the genital rudiment and other mesodermic elements : these cells 



