24 THE MOLLUSC A 



cndoderm and gradually surround it, leaving at the nutritive pole 

 an orifice, which is the blastopore. Examples of this mode of 

 formation are Many streptoneurons Gastropoda (Trochus, Vermetus, 

 Crepidula, Fig. 11, Janthina), the majority of the Rachiglossa 

 (Columbella, Fusus, Nassa, Purpura, Urosalpinx), the Tectibranchs 

 (Acera, Pkiline, Aplysia, Thecosomata), and many Lamellibranchs 

 (Pecten, Modiolaria, Cardium, Teredo, etc.). 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, etc., 

 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 : Nassa, 

 Purpiim, Fiung, etc. ; Tectibranchia : Accra, Aplysia, Carolinia, etc.), 

 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 Nasm 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 Molluscs the liver 



