STRUCTURE AND DEVELOPMENT OF VERTEBRATE OVARY; 425 



— about 0'38 mm. as compared with 0"22 mm. It is divided into 

 three obvious layers : (1) an outer epithelial layer which corre- 

 sponds with the pseudo-epithelial layer of the Elasmobranch ovary, 

 average thickness 0'03 mm. (3) A middle layer of small nests, 

 which corresponds with the middle vascular layer of the previous 

 stage; average thickness O'l min. (3) An inner layer of larger 

 nests; average thickness 0*33 mm. 



The general appearance of the germinal epithelium at this 

 stage certainly appears to me to lend support to my view that 

 the whole of it simply constitutes a thickened epithelium inter- 

 penetrated with ingrowths of stroma. 



The cells of the germinal epithelium, which form the various 

 layers, have undergone important modifications. In the first place 

 a large number of the nuclei — at any rate of those cells which are 

 about to become ova — have undergone a change identical with 

 that which takes place in the conversion of the primitive into the 

 permanent ova in Elasmobranchs. The greater part of the con- 

 tents of the nucleus becomes clear. The remaining contents 

 arrange themselves as a deeply staining granular mass on one side 

 of the membrane, and later on as a somewhat stellate figure : 

 the two stages forming what were spoken of as the granular and 

 stellate varieties of nucleus. To avoid further circumlocution I 

 shall speak of the nucleus undergoing the granular and the stel- 

 late modifications. At a still later period the granular contents 

 form a beautiful network in the nucleus. 



The pseudo-epithelium (fig. 38a) is formed of several tiers 

 of cells, the outermost of which are very columnar and have less 

 protoplasm than in an earlier stage. In the lower tiers of cells 

 there are many primitive ova with granular nuclei, and others 

 in which the nuclei have undergone the granular modification. 

 The primitive ova are almost all of the same size as in the earlier 

 stage. The pseud-epithelium is separated from the middle layer 

 by a more or less complete stratum of connective tissue, which, 

 however, is traversed by trabeculse connecting the two layers of 

 the epithelium. In the middle layer there are comparatively few 

 modified nuclei, and the cells still retain for the most part their 

 earlier characters. The diameter of the cells is about 0"012 mm., 

 and that of the nucleus about 0*008 mm. In the innermost 

 layer (fig. 38b), which is not sharply separated from the middle 

 layer, the majority of the cells, which in the previous stage were 

 ordinary cells of the epithelium, have commenced to acquire modi- 

 fied nuclei. This change, which first became apparent to a small 

 extent in the young two days after birth, is very conspicuous at 

 this stage. In some of the cells the nucleus is modified in the 

 granular manner, in others in the stellate, and in a certain 



