58 CRANIATA. 



becomes a beautiful reticulum, of the character so well known in 

 nuclei (fig. 19, do}. Two or three special nucleoli are present, 

 and form the nodal points of the reticulum, while its meshes are 

 filled up with the clear fluid constituents of the nucleus. Not all 

 the nuclei undergo the above changes ; but some of them stop 

 short in their development, undergo atrophy, and appear finally 

 to be absorbed as pabulum by the protoplasm of the nest. Such 

 nuclei in a state of degeneration are shewn in fig. 19. Thus only 

 a few nuclei out of a nest undergo a complete development. At 

 first the protoplasm of the nest is clear and transparent, but as 

 the nuclei undergo their changes the protoplasm becomes more 

 granular, and a specially large quantity of granular protoplasm 

 is generally present around the most developed nuclei, and these 

 with their protoplasm gradually become constricted off from the 

 nest, and constitute the permanent ova (fig. 19, do). The rela- 

 tive number of ova which may develop from a single nest is 

 subject to great variation. The object of the whole occurrence 

 of the fusion of primitive ova and the subsequent atrophy of 

 some of them is to ensure the adequate nutrition of a certain 

 number of them. 



In the second and rarer mode of development of permanent 

 ova from primitive germinal cells, the nuclei and protoplasm 

 undergo the same changes as in the first mode, but the cells either 

 remain isolated, and never form part of a nest, or form part of a 

 nest in which no fusion of protoplasm takes place, and in which 

 all the cells develop into permanent ova. 



The isolated ova and nests are situated, during the whole of 

 the above changes, amongst the general undifferentiated cells of 

 the germinal epithelium, but as soon as a permanent ovum be- 

 comes formed the cells adjoining it arrange themselves around it 

 as a special layer, and so give rise to the epithelium of the follicle 

 (fig. 19, o}. The growths of stroma into the germinal epithelium 

 appear shortly after the formation of the earlier follicles. 



Mammalia. The development of the ovary in Mammalia 

 differs mainly from that just described in that the formation of 

 primitive germinal cells from the indifferent cells of the germinal 

 epithelium takes place at a relatively much later period. 



The stroma grows into the germinal epithelium while it is still 

 formed of rounded indifferent cells, and divides it into trabeculae 



