346 THE BABBIT. 



A thin layer of connective tissue, the tunica albuginea, 

 extends parallel to the surface of the ovary, and divides the 

 germinal epithelium into two almost completely separate layers ; 

 a thin outer layer of columnar cells, investing the outer surface 

 of the ovary ; and a much thicker, deeper layer of spherical cells, 

 broken up into nests hy the connective tissue partitions. In the 

 germinal epithelium, and more especially in its outer layer, the 

 primitive ova are conspicuous, as individual epithelial cells, 

 much larger than their neighbours, and usually spherical or 

 polygonal in shape, with large granular nuclei. The tubuliferous 

 tissue is still present along the axis of the ovary, but now 

 occupies a relatively much smaller space than before. 



The permanent ova. Four or five days after birth of the 

 young rabbit, the germinal epithelium undergoes further 

 changes, marking the establishment of sexuality, and the con- 

 version of the genital ridge of the embryo into the definite 

 ovary. 



The changes consist essentially in the formation of per- 

 manent ova from the primitive ova, and occur in the rabbit in 

 much the same way as in the chick or tadpole. The nucleus, 

 or germinal vesicle, increases in size and becomes vesicular, 

 acquiring a very distinct nuclear membrane ; the nuclear con- 

 tents collect to one spot, where they form a granular mass, 

 from which, by branching, a definite reticulum is established ; 

 and, finally, one or more of the nodal points of the reticulum 

 enlarge, to form the nucleoli or germinal spots. 



The other cells of the germinal epithelium have small nuclei 

 and soon arrange themselves in a more or less definite manner, 

 so as to form follicles surrounding the permanent ova (Fig. 

 133, ga). 



At first (Fig. 188, oz), each nest of epithelial cells may con- 

 tain several permanent ova, but as development proceeds, and as 

 the follicle cells become more definitely arranged around the 

 ova, the nests are broken up by further ingrowth of the vascular 

 connective tissue, and the separate follicles isolated from one 

 another. 



Although it appears to be the rule in the rabbit's ovary that 

 each primitive ovum should become a permanent ovum, yet this 

 is by no means always the case. Sometimes two or more primitive 



