58 VERTEBRATE REPRODUCTIVE CYCLES 



follicle cells, and between the follicles are numbers of 

 interstitial cells similar to those in the testis. 



There are two types of vertebrate ovary which are 

 distinguished by the different degrees of development of 

 the eggs and follicles. In the egg-laying vertebrates the 

 egg is large, due to the inclusion of masses of yolk, and 

 the investing follicle is thin. In the marsupial and 

 eutherian mammals, on the other hand, the egg is 

 microscopic, since it contains no yolk, and it is the 

 follicle that is large. In all ovaries the sheath of follicle 

 cells must burst when an egg is shed. In the lower 

 vertebrates this involves the simple tearing of a thin sheet 

 of cells, but in the higher mammals the follicle is far 

 bigger than the egg which it encloses. Such a follicle 

 becomes distended not by the egg but by the develop- 

 ment of a cavity, the antrum, into which fluid is secreted. 

 Follicles with antra are called Graafian follicles, and 

 when fully charged with fluid they burst to release the 

 egg into the body cavity. 



After the bursting of a follicle the disorganized cells 

 reform into a group and swell into a structure known as 

 the corpus luteum. This structure is particularly obvious 

 in the mammals where its function has been closely 

 studied. 



The cycle of egg production in the ovary starts with 

 the division of one of the cells of the germinal epithelium. 

 Of the two cells thus formed, one passes into the ovary 

 by penetrating the connective tissue.^' ^^' ^2, 54, 57 

 Within the ovary it is known as an oogonium, it acquires 

 a sheath of follicle cells, and it grows without division 

 into a primary oocyte. In the egg-laying vertebrates the 

 main function of the follicle cells is to pass yolk into the 

 primary oocyte until it is greatly distended, but in higher 

 mammals their function is to pass fluid into the antrum. 

 Apparently in all vertebrates it is common for the final 

 divisions of the primary and secondary oocytes (with 

 the shedding of polar bodies) to take place in the oviduct 

 after ovulation. 



