24 THE MOLLUSCA 
endoderm and gradually surround it, leaving at the nutritive pole 
an orifice, which is the blastopore. Examples of this mode of 
formation are—Many streptoneurous Gastropoda (7rochus, Vermetus, 
Crepidula, Fig. 11, Janthina), the majority of the Rachiglossa 
(Columbella, Fusus, Nassa, Purpura, Urosalpinz), the Tectibranchs 
(deera, Philine, Aplysia, Thecosomata), and many Lamellibranchs 
(Pecten, Modiolaria, Cardium, Teredo, ete.). The two processes, 
however, differ only in appearance, and there are intermediate 
stages which form an insensible passage from one method to the 
other. In fact, complete invagination only occurs when the 
segmentation is quite or very nearly regular (Paludina, Chiton, ete., 
Figs. 10, A, and 110, A), but in consequence of the progressive 
increase of the amount of food-yolk contained in them, the macro- 
meres become larger and larger and are only able to be invaginated 
at a late stage of development. That is to say, in certain embolic 
gastrulae there is a commencement of epiboly, followed eventually 
by an invagination of the macromeres (Firoloida, Clione, Nucula). 
In the various cases enumerated above the segmentation of the 
ovum is complete or holoblastic. In the Cephalopods the case is 
different, for the segmentation is incomplete or meroblastic (Fig. 
289), a large part of the egg being formed of food-yolk which takes 
no part in the division. But it must be remarked that in various 
types, such as the specialised Gastropods (Rachiglossa: Nuassa, 
Purpura, Fusus, ete. ; Tectibranchia: Aceru, Aplysia, Cavolinia, ete.), 
there is a sort of quasi-distinct yolk, formed by the granular 
portion of the macromeres. Hence the meroblastic or “ discoidal ” 
segmentation of the Cephalopods is not absolutely distinct from 
the total segmentation observed in other Molluscs: it is only an 
exaggeration of epiboly. In fact, as the yolk forms the principal 
part of the ovum and the protoplasm is concentrated at the 
formative pole, the ectoderm is formed over a limited region of the 
yolk (the “ germinal disc” or “embryonic area”), and is unable to 
envelop it entirely, so that development proceeds as if the process 
of epiboly had been left incomplete, the blastopore remaining very 
large and leaving all that part of the yolk which could not be 
covered by the ectoderm outside the embryo. Under these 
circumstances the endoderm is essentially an embryonic tissue, 
exclusively employed in the constitution of the vitelline mass, and 
degenerates in the adult, a great part of the digestive tract of the 
latter, a long stomodaeum and a long proctodaeum, being formed 
by the ectoderm. The passage to this condition is presented by 
some Gastropods with an abundant yolk: in Massa a part of the 
primitive endoderm degenerates in the adult, and in Fusus the four 
macromeres of the primitive endoderm seem to form a provisional 
embryonic organ, and it is the ectoderm that forms nearly the 
whole digestive tube. In the different groups of Molluses the liver 
