THE MOLLUSC A 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 (Ci/d(ts, Ureissensia, 

 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, M). 



Formation of the Digestive Cavity and <>f the Diulastula 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 



FIG. 10. 



Two types of gastrulae. .4, invaginate or enibolic (Cliitoii : after Kowalewsky) ; /,', epibolic 

 (Crepidvla : after Conklin). bl, blastopore ; ec, ectoderm ; en, entodenn. 



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, much as 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 

 imagination 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. 1 0, A), Paludina, the Pulmonata, the 

 Xudibranchia, the Pteropoda, the Gymnosomata and Limacinidae, 

 the Scaphopoda, Nacnla, Ostmcu, 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 in vagina ted into the layer of micromeres. In 

 such case the micromeres as they multiply grow round the 



