472 



ported in many groups and as often denied. 

 For a survey of the general status of the 

 germ cell theory see Heys ('31); for more 

 recent discussions see Willier ('39), Nieuw- 

 koop ('47, '49), Gillman ('48), Witschi ('48) 

 and Johnston ('51). There is as yet no ade- 

 quate experimental analysis for the early 

 stages of development. But if the ultimate 

 source of the germ cells in most cases re- 

 mains in doubt, it is generally accepted that 

 they originate outside the gonad-forming 

 area. 



Source of the Primordial Germ Cells. In 

 only one case is there significant evidence 

 indicating early segregation of a germinal 

 material in a vertebrate. In the egg of a frog 

 a cytoplasmic substance in the yolk near the 

 vegetal pole has been traced to the "yolk 

 ridge" of the gut (Bounoure, '34), the site 

 where the germ cells are generally identified 

 in anurans. Irradiation of the vegetal pole 

 results in gonads which are sterile (see 

 Bounoure, '50). In urodele eggs, on the other 

 hand, elimination or addition of polar yolk 

 material does not seriously affect the de- 

 velopment of germ cells (Nieuwkoop, '47, 

 '50). 



In most vertebrates the germ cells as such 

 have first been identified in various parts of 

 the entoderm: the "yolk ridge" of anuran am- 

 phibians, the entoderm of the yolk sac in rep- 

 tiles, the splanchnopleure of the proamnion 

 region of the avian blastoderm, and the hind 

 gut or adjacent yolk sac entoderm in mam- 

 malian embryos. Urodele amphibians are a 

 notable exception. In this group most in- 

 vestigators have first identified the germ cells 

 in the "intermediate mesoderm" of the pos- 

 terior tnmk (see Humphrey, '25, '29). A 

 recent experimental analysis (Nieuwkoop, 

 '47, '50) shows that in late yolk-plug and 

 neurula stages they occupy a corresponding 

 position in the prospective lateral mesoderm, 

 ventral and ventrolateral to the blastopore 

 (Fig. 170/4). Removal of this area in the 

 early neurula results in sterility. Neverthe- 

 less, the entoderm is in some way involved in 

 their differentiation as germ cells. In its ab- 

 sence germ cells fail to appear in the gonad, 

 or are greatly diminished in number. It is 

 concluded that precursor cells in the prospec- 

 tive lateral mesoderm are able to differen- 

 tiate into germ cells only after contact with 

 the dorsocavidal entoderm (Nieuwkoop). 



In avian embryos the germ cells are 

 usually said to lie in the extra-embryonic 

 splanchnopleure, near the germ wall In the 

 chick they tend to be concentrated anteriorly 

 in the "germ cell crescent" (see Swift, '14). 



Special Vertebrate Organogenesis 



Destruction of the crescent or isolation of the 

 gonad-forming area of the early blastoderm 

 both result in genital ridges or gonads which 

 lack germ cells (for a discussion see Willier, 

 '39, '50). The evidence svipports the view 

 that the cells in question are indeed pri- 

 mordial germ cells but it is hardly crucial. 



Migration of the Primordial Germ Cells. 

 If the germ cells originate outside the gonad 

 region, sometimes at a considerable distance, 

 there is a problem as to how they reach their 

 destination in the genital ridge. In urodele 

 amphibians the situation is fairly simple. 

 From the earliest known stages, the germ 

 cells lie with the other gonad constituents in 

 the lateral plate mesoderm, and appear to 

 move passively with the lateral plate ma- 

 terials to a point medial to the Wolffian duct 

 (Humphrey, '25; Nieviwkoop, '47). For entry 

 into the genital ridge, independent "amoe- 

 boid movement" directed by an influence 

 emanating from the ridge has been sug- 

 gested. In anurans, according to Bounoure 

 (above), the germinal material is carried 

 from the vegetal pole to the "yolk ridge" by 

 the movements of gastrvilation and dorsal 

 closure of the gut. The "yolk ridge" is then 

 separated from the gut by the growth forces 

 which produce the mesentery, after which 

 only a slight lateral shift to the genital ridge 

 is necessary. For this, independent migration 

 and a directive influence from the ridge are 

 again postulated. 



In amniote embryos migration from the 

 yolk sac or gut entoderm involves more 

 difficulty. In most cases independent migra- 

 tion through the splanchnopleural mesen- 

 chyme is assumed, based on the distribution 

 of germ cells from stage to stage, and the 

 appearance of "pseudopodial" form (e.g., see 

 Witschi, '48). However, passive transport 

 may again be a factor locally, as when germ 

 cells embedded in the coelomic epithelium 

 (prospective germinal epithelium) are shifted 

 around the dorsal angle of the body cavity 

 (see Willier, '39). The most unusual "migra- 

 tion" is described in bird embryos, where 

 the germ cells presumably move from the 

 "germ cell crescent" to the gonad region. 

 According to the classic theory of Swift ('14) 

 they are enveloped by extra-embryonic blood 

 vessels and enter the embryo, some escaping 

 in the gonad region. Definite proof of this 

 hypothesis is lacking. Destruction of the 

 crescent area, or early isolation of the gonad- 

 forming region, both result in sterility, 

 indicating that the germ cells have been 

 eliminated or excluded. However, this re- 

 sult sheds little light on the pathway or 



