THE CEPHALOPODA 
Oo 
No 
On 
Ill. Empryouocy. 
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, pr ovided - 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, vz) the quantity of yolk is less than 
bE : in the other members of the order, and that 
“0 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, bi), 
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 
Fic, 289. which it appears to be seated (Fig. 291). 
Egg of Loligo in the first Lhe extent of the embryonic area and of 
Segmentation stage. i, the the free surface of the yolk are in inverse 
vitellus. (After Watase.) ratio to one another: the external vitelline 
mass is smaller in Loligo 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 
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