176 



Embryogenesis: Preparatory Phases 



those of others, as definitive proof of the ex- 

 tragonadal origin of all of the germ cells in 

 the chick. He points to three somewhat 

 anomalous results: (1) Sterile gonads may 

 be obtained from grafts that contain the 

 germinal crescent as well as from those in 

 which that area has been excluded. (2) 

 Sterile gonads may also form from grafts of 

 the gonad-forming area taken at a stage 

 when it contains germ cells. (3) Germ cells 



Dorsal 



Ventral 



Fig. 52. Diagram illustrating the location of the 

 presumptive primordial germ cells in urodeles at 

 the early yolk plug stage according to experiments 

 of Nieuwkoop ('46). d.l.. Dorsal lip of blastopore; 

 P, area of presumptive lateral plate and nephrog- 

 enous cord mesoderm containing the presumptive 

 primordial germ cells. 



may appear in grafts of Hensen's node 

 taken at the head process stage. Willier 

 suggests that results 1 and 2 may be at- 

 tributed to lack of development of a germ- 

 cell transporting mechanism (blood vascular 

 system) in the graft and to lack of some 

 condition in the graft that is essential for 

 proper growth of the gonad. Result 3 may 

 be interpreted on the basis of observations 

 that show the presence of some primordial 

 germ cells in regions posterior to the antero- 

 lateral crescentic zone of the area pellucida 

 that had previously been considered to be 

 their locus. While the experiments are not 

 presented as definitive proof of the extra- 

 gonadal origin of the primordial germ cells 

 in birds, they offer strong support for that 

 view. 



Evidence that the mature sex cells are 



derived from the primordial germ cells 

 rather than secondary germ cells is provided 

 by experiments on removal of the left ovary 

 of young chicks. As is well known, the 

 rudimentary right ovary then tends to 

 develop into a testis. Normally the primor- 

 dial germ cells of the rudimentary right 

 gonad disappear after the third week. If 

 removal of the left ovary is performed prior 

 to this disappearance the right gonad may 

 form a testis with mature spermatozoa, 

 whereas upon later removal no spermatozoa 

 are produced in the testis that develops 

 (Domm, '29). 



Aniirans and urodeles apparently differ in 

 regard to the site in which cells that have 

 the appearance of primordial germ cells 

 are first found. In the former it is the ento- 

 derm of the gut wall; in the latter it is 

 the lateral plate mesoderm. The results of 

 experiments in these two groups seem, on 

 the whole, to be consistent with the observed 

 differences in location of such cells. Thus, 

 in the anuran Discoglossus, Monroy ('39) 

 obtained germ cell-free embryos when the 

 ventral part of the entoderm was removed 

 from caudal halves of early nemrulae, while 

 removal of the dorsal entoderm did not alter 

 the number and location of the germ cells. 

 However, Monroy's experiments do not ex- 

 clude the interpretation, proposed by Nieuw- 

 koop ('46) for salamanders, that the ventral 

 entoderm acts as an inductor on "predis- 

 posed" primordial germ cells that originate 

 elsewhere. 



In urodeles the most extensive experiments 

 appear to be those of Humphrey ('27, '28, 

 '29, '33) and Nieuwkoop ('46). By means 

 of a large variety of extirpation and trans- 

 plantation experiments Nieuwkoop has con- 

 firmed Humphrey's view that the germ cells 

 originate in the presumptive lateral plate 

 mesoderm which, in the gastrula, is repre- 

 sented by the ventral and ventrolateral lips 

 of the blastopore. In the uncleaved egg this 

 material would be located equatorially op- 

 posite the grey crescent rather than at the 

 vegetal pole that Bounoure regards as its 

 position in anurans. Nieuwkoop was able 

 to distinguish germ cells of different species 

 by differences in their content of pigment 

 granules. So heteroplastic transplantation 

 could be vised to demonstrate that the pre- 

 sumptive lateral plate mesoderm is the only 

 soiorce of germ cells (see Fig. 52). However, 

 when the presumptive dorsocaudal entoderm 

 is removed at the gastrula stage, or the 

 whole entoderm at the neurula stage, there 

 is a considerable reduction in the number of 



