XII 



PHYLUM MOLLUSCA 



699 



times 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 is sometimes embedded in 

 gelatinous matter enclosed in a firmer envelope, and a number of 

 them are arranged in a string ; sometimes a large number are em- 

 bedded in a rounded gelatinous mass. Usually, as in 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 excavated in the 

 earth. 



In a few marine and 

 fresh- water Gastropoda the 

 ova undergo their develop- 

 ment in the body of the 

 mother, enclosed in an 

 enlargement of the ovi- 

 duct 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 

 l>e distinguishable. There 

 is a fairly close agreement 

 throughout the class in 

 the nature of the segmen- 

 tation (Fig. 60G). In all 

 eases it is total, sometimes 

 equal at first, but soon 

 afterwards becoming un- 

 equal. The first four 

 blastomeres are usually 

 equal or nearly so; they 



are so arranged that two of them are in contact in the middle, 

 and thus separate the other two : the line of contact of the 

 former pair becomes the transverse axis of the embryo. 



From the four first-formed cells four small cells micromeres 

 become' constricted off, the larger cells being the megameres; 

 then four more micromeres are constricted off, and again the 

 same process is repeated. The embryo now consists of the four 

 megameres and twelve micromeres. The latter then increase by 

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

 the megameres. The whole process, as will be noticed, has a 



FIG. C05. Forms of egg cases in Gastropoda. A and 

 J), Pyrula or Busycon J B, Conus ; C, 

 Voluta musica; E, Ampullaria. (From 



iji. y<(ti'.i-<U History.) 



