480 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



which consists of eight spermatoblasts, with a 

 small central mass of inactive protoplasm (folasto- 

 phore). Division of the cells still continues, until at 

 last we get a spermatosphere, which consists of a 

 number of elongated spermatoblasts supported by the 

 blastophore (Fig. 203 ; c). The protoplasm around 

 the nucleus of each spermatoblast next collects into a 

 small cap, and then gives off' a delicate filamentous 

 process (Fig. 203 ; D), which, gradually increasing in 

 size, comes to form the tail of the spermatozoon. 

 Further changes in form are effected, and the con- 

 stituent spermatoblasts of the sphere fall away from 

 one another, to become, each of them, an actively 

 motile spermatozoon capable of fertilising a female 

 cell. 



The essential points in this history have been 

 detected by various observers in other animals, many 

 of whom have, however, somewhat obscured the 

 subject by the number of new technical terms which 

 they have introduced. 



Oogenesis, or the development of ova, has 

 been more thoroughly studied than spermatogenesis, 

 but the subject is rendered more complicated by the 

 fact that the egg cell either absorbs in early periods, 

 or is for a time surrounded by nutrient or yolk cells. 

 The egg cells of the earthworm form a coherent mass, 

 which occupies a similar position in the thirteenth to 

 that occupied by the testes in the tenth and eleventh 

 segments, and is only distinguished by the investment 

 of firm membrane, which surrounds the mass of cells, 

 or ovary, and separates it from the rest of the 

 epithelium of the body cavity ; the constituent cells 

 of this ovary do not, however, undergo the segmenta- 

 tion which affects the male elements. 



Consisting, in the simplest cases (e.g. Hydra), of a 

 naked mass of protoplasm, the ovum, with its nucleus 

 (here called germinal vesicle) and nucleolus (or 



