802 



SPERM, OVA, AND PREGNANCY 



fortunately, superovulation in cattle which 

 has been achieved by the administration of 

 gonadotrophic hormones (Casida, Meyer, 

 McShan and Wesnicky, 1943; Umbaugh, 

 1949; Hammond, 1950a, b) has met with 

 little success as a means of inducing preg- 

 nancy (Willett, Black, Casida, Stone and 

 Buckner, 1951 ». 



III. Biology of the Mammalian Egg 



A. OOGENESIS 



The literature is now revealing a more 

 clear cut opinion as to whether or not the 

 primordial germ cells from the yolk sac of 

 the embryo are set aside at the beginning of 

 ontogenesis, or whether they arise de novo 

 from the somatic cells of the gonadal peri- 

 toneum in the embryo and particularly the 

 sexually mature female. Knowledge in this 

 field has been significantly advanced by em- 

 ploying the techniques of experimental em- 

 bryology, organ and tissue culture, histo- 

 chemistry, x-rays, ultraviolet irradiation, 

 genetics and statistics. The Gomori alkaline 

 phosphatase procedure lias been used by a 

 number of investigators to distinguish selec- 

 tively the primordial germ cells in the hu- 

 man (McKay, Hertig, Adams and Danzigcr, 

 1953), the mouse (Chiquoinc, 1954; Mintz, 

 1959), and the rat (McAlpine, 19551. Using 

 the same technique, Bennett (1956) reported 

 the absence of germ cells in strains of mice 

 known to be sterile. It has been suggested 

 that the high alkaline phosphatase activity 

 in the germ cells may be related to their ac- 

 tive movement through tissues. This specu- 

 lation has merit when it is noted that alka- 

 line phosphatase activity is greatly reduced 

 in amblystoma, in which the germ cells do 

 not actively migrate, and in the chick where 

 these cells are apparently transported by 

 way of the blood stream (Chiquoine and 

 Rothenberg, 1957, Simon, 1957a, b). It 

 should be noted that the primordial germ 

 cells may be identified by other techniques. 

 For example, in the rat and man the use of 

 the periodic acid-Schiff (PAS) reaction and 

 a hematoxylin counter stain gives such 

 excellent cytologic differentiation of the 

 germ cells that they can be counted and 

 their migratory course followed (Roosen- 

 Runge, personal communication). 



It is beyond the scope of our discussion 



to present the details of the controversy of 

 germ cell origin, migration, localization, and 

 proliferation. Excellent reviews of the better- 

 known theories are contained in the papers 

 and monographs of Heys (1931), Cheng 

 (1932), Swezy (1933), Pincus (1936 », 

 Bounoure (1939), Everett (1945), Nieuw- 

 koop (1949), Zuckerman (1951), Brambell 

 (1956), and Nieuwkoop and Suminski 

 ( 1959) . Evidence for the extragonadal origin 

 of the primordial germ cells has been signifi- 

 cantly enhanced by the more recent investi- 

 gations in amphibia, birds, and various 

 mammals such as the armadillo, mouse, rat, 

 cat, rabbit, and man. In an excellent paper 

 dealing with the migration of the germ cells 

 in the human, Witschi (1948) points out that 

 in embryos of less than 16 somites all of the 

 primitive germinal elements are located in 

 the endoderm of the yolk sac splanchno- 

 l)leure near the site of evagination of the 

 allantois (Fig. 14.3). From this location the 

 individual germ cells appear to migrate to 

 the genital folds by various routes. Witschi 

 concludes from studies of sectioned human 

 embryos that the migration of the germ 

 cells is accomplished by active autonomous 

 movements and cites evidence of proteolysis 

 of the cells and tissues in the immediate 

 vicinity of the forward moving cells. He 

 suggests that the specific orientation of the 

 cell is directed by some chemical substance 

 released by the peritoneum of the gonadal 

 regions. 



A very important contribution to the 

 solution of the problem of seeding the primi- 

 tive gonads by germ cells from extragonadal 

 origin is described in the contributions of 

 IVIintz (1957, 1959) and Mintz and Russell 

 (1957). These authors noted that the gonads 

 of mice of the WW, WW'' and WW^ geno- 

 tyi^es are almost devoid of germ cells at 

 birth. The application of the alkaline phos- 

 phatase technique revealed that the cells 

 are present in their usual numbers in the 

 yolk sac splanchnopleure by the 8th day 

 of development. The mutant genes appar- 

 ently do not impair the initial formation of 

 the primordial germ cells. By the 9th day 

 of development, however, many of the germ 

 cells had already degenerated at their site 

 of origin. Some of them escape destruction 

 and migrate toward the genital ridge. The 

 migratory cells fail to divide so that the 



