REPRODUCTIVE SYSTEM. 105 



more spermatozoa. These may be lost within investing mem- 

 brane, or gain access to the germinative area through a 

 special opening (micropyle). 



The first evidences of impregnation are disappearance of 

 the germinal vesicle and spot, when present,* and the seg- 

 mentation of the vitellus, forming the mulberry mass. Seg- 

 mentation may be partial, as in Aves, or complete, as in 

 Annelides. 



The unsegmented yolk is designed for the nourishment of 

 the embryo during incubation, and bears fixed relations to 

 the conditions surrounding the impregnated egg and to the 

 time occupied in the evolution of the embryo. Where ova 

 are retained in connection with, and are nourished through 

 the tissues of the mother (gestation), as in Mammalia, or 

 where they are early discharged from the mother, and devel- 

 opment to a self-supporting stage proceeds rapidly, as in Pro- 

 tozoa, Radiata, Entozoa, and most Mollusca, little or no vitel- 

 lus is required. Such examples compose the first group of 

 eggs, characterized as follows: Eggs, for the most part of 

 small size, often very minute; the yolk substance entirely 

 composed of elementary granules or minute and simple 

 spherules; the process of segmentation affecting the whole 

 mass of the yolk, and the germinal layer, resulting from that 

 segmentation, extending from the first over the whole sur- 

 face of the ovum. The whole yolk is germinal, or con- 

 verted into the parts of the future embryo. In the second 

 group, the great majority of which are oviparous, the ova are 

 proportionately of the largest size. The yolk substance con- 

 sists very obviously of two kinds of organized particles, viz., 

 of small granules, nearly similar to those which form the 

 whole yolk in the last group, and which alone undergo seg- 

 mentation, large cells, usually non-nucleated, and fat vesi- 

 cles, which constitute the greater part of the mass. The first 

 of these enter into the germinative area, the second the 

 part making up the great bulk of the egg is strictly nutri- 



* Rarely the germinal spot and vesicle are not lost after conception, 

 but the germinal vesicle itself undergoes segmentation, as in Ascaris mystax. 

 (Nelson.) 



