730 



ZOOLOGY 



SECT. 



many cases only a limited number sometimes only one of the 

 embryos contained in the capsule become developed, the rest 

 serving as nutriment for the survivors. 



In the land Pulmonata each ovum may be embedded in gelatinous 

 matter enclosed in a firmer envelope, and a number of them are 

 arranged in a string ; sometimes a large number are embedded 

 in a rounded gelatinous mass. Usually, as in some species of 

 Helix and other genera, the outer layers of the albumen-like 

 substance enclosing the egg become toughened and impregnated 

 with salts of lime, so as to assume the character of a calcareous 

 shell ; a number of such eggs, which are of relatively considerable 

 size, are laid in holes exca- 

 vated in the earth. 



In a few marine and 

 fresh-water Gastropoda the 

 ova undergo their develop- 

 ment in the body of the 

 parent, enclosed in an en- 

 largement of the oviduct 

 which serves as a uterus. 



The egg contains a con- 

 siderable quantity of food- 

 yolk, which may be evenly 

 distributed, or a clear pro- 

 toplasmic and an opaque 

 yolk-laden segment may be 

 distinguishable. There is 

 a fairly close agreement 

 throughout the class in the 

 nature of the segmentation 

 (Fig. 653), which is very 

 similar in the early stages 

 to that of Nereis and the 



PWtrmnrla in frpnprnl (r> FlG - 652. Forms of egg-cases in Gastrgpoda. A and 



erai (p. ^ pyrula or Busycon; B, Conus; C, 



440) and to that Of a Poly- Voluta musica; E Ampullaria. (From 



i -i / r/r>\ T 11 the Cambridge Natural History.) 



clad (p. 268). In all cases 



it is total, sometimes equal at first, but soon afterwards becoming un- 

 equal. The first four blastomeres, A, B, C, D (cf. Fig. 219) are 

 usually equal or nearly so. From these a succession of four quar- 

 tettes of smaller cells (micromeres) become divided off, the larger 

 cells being the megameres. The former then increase by division 

 and form a cap of small cells (ectoderm) on the surface of the 

 megameres. Of the cells of the fourth quartette, three become 

 endoderm cells, contributing to the endodermal lining of the 

 mesenteron. The fourth becomes the parent cell of the mesoderm. 

 The megameres themselves eventually become converted into 

 endoderm cells with the exception of D, which, before becoming 



