BIOLOGY OF EGGS AND IMPLANTATION 



807 



formation that has been recorded for the 

 amphibia and invertebrates (Tyler, 1955). 

 Heteroplastic transplantations and other 

 experimental procedures which can be per- 

 formed more easily in these animals may 

 lead to explanations of the fundamental 

 patterns of germ cell-inducing influences by 

 the surrounding cells and to other problems 

 bearing on the question of the origin of 

 second generation germ cells in the genital 

 ridge. 



B. GROWTH, COMPOSITION, AND SIZE OF 

 THE MAMMALIAN EGG 



The rate of growth of the oocyte in re- 

 lation to the stage of development of the 

 ovarian follicle has been investigated in a 

 numl)er of placental mammals (Brambell, 

 1928, mouse; Parkes, 1931, rat, ferret, rab- 

 l)it, pig; Zuckerman and Parkes, 1932, ba- 

 boon; Green and Zuckerman, 1951a, 1954, 

 Macaca mulatta and man). The available 

 information indicates that size relationship 

 of ovum and follicle has the same c^uantita- 

 tive aspect in all animals studied. It is in- 

 teresting that the regression line relating to 

 the size of egg and follicle is steep in the 

 first phase and almost horizontal in the 

 second (Fig. 14.4). It is generally believed 

 that the ovum attains its mature size about 

 the time antrum formation begins in the 

 follicle. Further, it is also believed that 

 follicular response to pituitary hormones is 

 confined primarily to those follicles in which 

 the ova have attained their full dimensions 

 (Pincus, 1936). It is well known that not 



all ova grow to mature size. Factors de- 

 termining which of the ovarian eggs are 

 destined to begin their growth or to com- 

 plete their growth during a reproductive 

 cycle are unknown and present very chal- 

 lenging problems. Growth of the follicle be- 

 yond the antrum stage may be quite in- 

 dependent of the presence of an ovum. This 

 has been demonstrated in a variety of ways, 

 but particularly by the observation that in 

 senile rats large anovular follicles are of 

 common occurrence (Hargitt, 1930). The 

 converse has been reported; ova may grow 

 to full size within the stroma of an ovary 

 without being invested by follicular cells. 



Of particular interest, also, are the ques- 

 tions raised by Gaillard (1950) and Dawson 

 (1951) of the histogenetic relationship be- 

 tween the oocyte and follicular cells and 

 the oocytic potentiality of the follicular cells 

 themselves. In tissue culture explants from 

 human fetal ovarian cortex, Gaillard de- 

 scribed the development of cord-like groups 

 of cells from the germinal epithelium. A 

 second group of cord-like outgrowths de- 

 veloped from the follicular cells of the pri- 

 mordial follicles in which the oocytes had 

 degenerated. New oocytes developed within 

 these follicular cords and the surrounding 

 cuboidal epithelial cells arranged themselves 

 in a single layer to form the corona radiata. 

 The observations of Gaillard emphasize the 

 potential histogenetic interrelationships be- 

 tween the egg and the first layer of follicular 

 cells. The possible inductive relationships of 

 the ovarian egg and the various components 



J I I L 



J I 



10 20 30 40 50 60 70 80 90 gg 600 8001000 2000 3000 4001 



Diameter of foMicle (/i) 



Fig. 14.4. Regression lines relating size of ovum and follicle in human ovaries (Green and 

 Zuckerman, 1951b). 



