750 ZOOLOGY SECT. 



float about freely. In the Streptoneura (Fig. 642), instead of a 

 jelly-like mass, the eggs are enclosed in a firm parchment-like 

 capsule, in which is contained, in addition to the eggs, a quantity 

 of an albuminous fluid, serving to nourish the developing embryos. 

 The shape of the capsule varies greatly in the different genera : 

 sometimes it is stalked, sometimes sessile ; in some cases there 

 is a lid or operculum, the opening of which permits the embryos 

 to escape. Very commonly large numbers of these capsules are 

 aggregated together, and usually they are attached to a rock or a 

 sea-weed or similar object. In 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 em- 

 bedded 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 excavated in the earth. 



In a few marine and fresh-water Gastropoda the ova undergo 

 their development in the body of the parent, enclosed in an 

 enlargement of the oviduct which serves as a uterus. 



The egg contains a considerable quantity of food-yolk, which 

 may be evenly distributed, or a clear protoplasmic and an opaque 

 yolk-laden segment may be distinguishable. There is a fairly 

 close agreement throughout the class in the nature of the segmen- 

 tation (Fig. 643). In all cases it is total, sometimes equal at first, 

 but soon afterwards becoming unequal. 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 or micrcmeres 

 become constricted off, the larger cells being the megameres ; 

 then four more micromeres are divided 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 

 remarkably close resemblance to the process of segmentation of 

 the ovum of a Polyclad as described on p. 273. 



The megameres then give off internally four small endoderm 

 cells, and from one of these (endo-mesoderm cell) are formed two 

 primitive mesoderm cells, from which the cells of the mesoderm 

 are developed. In some cases (Paludina) the mesoderm is formed 



