THE MOLLUSCA 23 
is only a single macromere during the early stages of segmentation. 
The segmentation sphere or blastula is thus formed of two more 
or less unequal moieties, respectively known as the formative 
(ectodermic) and nutritive (endodermic) moieties. Internally there 
is a segmentation cavity or blastocoele, generally much reduced 
(Fig. 10, A), except in certain Lamellibranchs (Cyclas, Dreissensia, 
etc.) and stylommatophorous Pulmonates. The animal or formative 
pole of the egg is indicated by the presence of the polar bodies ; 
the vegetative or nutritive pole is opposite, and at this point the 
blastopore or orifice of the digestive cavity will be formed 
(Fig. 110, A, 2). 
Formation of the Digestive Cavity and of the Diblastula or Gastrula. 
—The final result of the segmentation is that the micromeres form 
a more or less complete envelope to the segmented ovum, covering 
over the macromeres which remain within to form the endoderm. 
But the formation of this double-walled sphere, the diblastula or 
Fic. 10. 
Two types of gastrulae. 4, invaginate or embolic (Chiton : after Kowalewsky) ; B, epibolic 
(Crepidula y after Conklin). 1, blastopore ; ec, ectoderm ; en, entoderm. 
gastrula, may be effected in one or the other of two apparently 
different methods, invagination or epiboly. Invagination or 
emboly is certainly the more primitive method, and is realised in 
ova with a relatively small amount of food-yolk. In this case the 
nutritive moiety of the blastula is doubled back within the 
formative moiety, muchas one half of a deflated hollow indiarubber 
ball may be pushed by the finger within the other half. These 
two halves are separated by a remnant of the original segmentation 
cavity, which is invariably much reduced by the process. The 
invagination thus produced gives rise’ to the digestive cavity or 
archenteron, lined by the endoderm and communicating with the 
exterior by the blastopore. As examples of this mode of formation 
one may cite Chiton (Fig. 10, A), Paludina, the Pulmonata, the 
Nudibranchia, the Pteropoda, the Gymnosomata and Limacinidae, 
the Scaphopoda, Nucula, Ostraca, Pisidium, and the Unionidae. 
The gastrula is formed by epiboly when the nutritive cells or 
macromeres have become so much distended with food-yolk as to 
be too large to be invaginated into the layer of micromeres. In 
such case the micromeres as they multiply grow round the 
