1910-11.] Development of Germ Cells in Mammalian Ovary. 153 
ment of the ovary is given. The main facts arrived at by an examination 
of the literature on the development of the ovary as a whole are that the 
growth of the germ and stroma cells is a mutual one ; also that the 
medullary cords are the first structures to be differentiated after the 
formation of the genital ridge. These cords have in their substance small 
epithelial cells, which, according to v. Winiwarter and Sainmont, have an 
atrophic influence on the surrounding cells. 
The confusion of terms between the tubes of Pfliiger, cortical cords, and 
cell nests is easily explained when it is remembered that the observations 
of the different workers were drawn from a study of very varied mammal- 
ian types. In the kitten, these germinal cell clusters have the appearance 
of radiating tubules, in the puppy they are in cell masses, while they are 
not clearly differentiated in the human foetal ovary, owing to the small 
amount of stroma present in that organ in its early stages. Although the 
different stages of development of the ovary occur at varying periods among 
the mammals, the human foetal ovary in its early stages showing the same 
growth as some other mammalian ovaries after birth, the plan of develop- 
ment as a whole can be accepted from a comparison of the various types. 
In the earlier accounts of the development of the ovary no mention of 
any length is made of the changes which occur in the nuclei of the germ 
cells, although Balfour has recognised some points of difference in these 
nuclei. The account of these changes must be sought for in the literature 
on the reproductive cells of plants and the lower animals, as the whole 
process of maturation can be followed from the earliest development of the 
sex cell up to fertilisation with almost mathematical precision. The favourite 
material for this observation is that of Ascaris, as in the mammalian types 
the early phases of maturation can only be followed, the ovum not 
being recognisable after its extrusion from the ovary until fertilisation 
takes place. 
In mitotic division of somatic cells there is no reduction in the number 
of chromosomes, but in the reproductive cells this reduction takes place in 
the course of the last two cell divisions, four cells being formed, and having 
half the usual number of chromosomes. The germ cell divides into four, 
but only one of these cells forms an ovum ; the other three form small cells 
which cannot develop, i.e. the polar bodies or “ abortive eggs ” (Mark (15)). 
The process of the formation of the polar bodies is given by Wilson (32) in 
detail. Yon Winiwarter in 1900 describes the changes which take place in 
the germ cells up to the formation of the resting nucleus or germinal 
vesicle. The earliest cells or oogonia are small, oval, and granular, with 
masses of chromatin, and without a nucleolus. These cells proliferate 
