92 ANIMAL PARASITES. 



receptacle (fertilization), there are in the uterus numerous clear, 

 round corpuscles, like the germ-granules, but somewhat larger, 

 and bearing on their outer surface a small aggregation of grains 

 (vitellus). Round both a delicate substance is seen deposited. 

 Thus the germinal grain (a sort of germinal vesicle) and germinal 

 corpuscle form a common oval mass. An egg-membrane, and a 

 germinal vesicle furnished with a germinal spot, and surrounded 

 by a vitellus, such as we usually find in the eggs of animals, 

 are therefore deficient in the eggs of the Cestodea. 



After the filling of the seminal receptacle, the germ-grain of 

 the cestoid egg breaks up by continual division into a mass of 

 small, round, clear vesicles or cells, which at first resemble the 

 germ-grain, and subsequently exhibit a paler envelope with a 

 nucleus. There is therefore no endogenous or new formation of 

 cells round nuclei in the egg, but division or segmentation. 

 The spheres of segmentation measure, after the first division 

 0'0095 millim. ; after the second, which occurs rather towards 

 the poles than in the middle of the egg, two larger and two 

 smaller spheres are formed, and we also subsequently find 

 unequal spheres, which at last become unmeasureable. Finally, 

 the aggregation of cells, which was previously blackberry-like, 

 becomes globular, and nearly homogeneous, and increases con- 

 siderably. The granular, vitellus-like mass, placed close to the 

 germinal grain or germinal vesicle, which lies beside the latter 

 like a cone or little roof, does not take part in this division ; the 

 enveloping mass is more distinct. There is consequently a 

 division analogous to the segmentation of the vitellus, which, 

 however, does not occur in the entire primitive egg, but only in 

 the gerrn-grain or germinal vesicle, and in which the small 

 aggregation of granules on the outer surface of the germ-grain is 

 not included. It has certainly been generally admitted of late, 

 that in the process of the segmentation of eggs, not only the 

 vitellus, but also the germinal vesicle divides. Moreover, in the 

 eggs of other animals also, the vitellus is sometimes deposited 

 not around, but beside the germinal vesicle, for example, in 

 Lizzia and Oceania. The principal distinction consists in the 

 want of the vitelline membrane, and the envelope round the 

 mature embryo is a new formation. The aggregation of cells 

 formed by division, and the granular vitelline mass, now 

 remain for a time unchanged in the common envelope ; some 

 cells then disappear, and numerous .nucleus-like granules remain. 



