THE ORIGIN OF THE DEFINITIVE OVA 29 



from the germinal epithelium. What then is the role of the 

 primordial germ cells of the embryo? Are they essential 

 structures or merely incidental? There are practically no 

 illuminating experimental data on the development of em- 

 bryonic gonads. The experimental manipulation of mamma- 

 lian embryos is dependent upon the elaboration of techniques 

 now in the process of initiation. Certain investigations 

 of gonadogenesis in amphibian and chick embryos offer 

 provocative suggestions, but their applicability to mammals 

 has yet to be proven. 



In the chick a gonad or gonad-like organ may form free of 

 primordial germ cells. This can be demonstrated by removal 

 or destruction in three to nine somite embryos of the anterior 

 crescent in which the primordial germ cells originate. The 

 embryos nonetheless develop small gonad rudiments (Rea- 

 gan, 1916; Benoit, 1930). Willier (1932, 1933a and h) has 

 excised the germ cell crescent and transplanted the entire 

 blastoderm and found a sterile gonad developed in the 

 transplant. In the frog (Kuschakewitsch, 1910) sterile 

 gonads free of germ cells develop from the genital ridge 

 when delayed fertilization prevents germ cell migration, 

 Humphrey (1928), on the other hand, finds that in Ambly- 

 stoma gonads form in grafted tissue only when a sufficient 

 number of primordial germ cells are located beneath the 

 coelomic epithelium which gives rise to the germinal epi- 

 thelium. ' 



It is notable that in all instances gonads arising free of 

 primordial germ cells are sterile. Thus Domm (1929) found 

 in the fowl that if the large functional left ovary is removed 

 prior to the time of the disappearance of the germ cells from 

 the small rudimentary right gonad the latter develops into 

 a testis which produces sperm. If excision of the left ovary 

 is delayed until the time when the germ cells of the right 

 gonad are no longer present (the germ cells normally dis- 

 appear by the third week after hatching) a sterile testis 

 develops. 



Willier (1933a and h) has demonstrated by means of 



