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BAUM ‘Ol have identified them in early stages of development among 
the birds. Janosık ’85 and MiınArkovics ’85 gave admirable reviews 
of the literature upon this subject, not only for the Sauropsida, but for 
the Vertebrates as a whole. 
In no species of Mammals has the study of their origin been car- 
ried back to such early stages as among the the lower Vertebrates. 
The reader may be referred to the works of Nagen ’89, ÜoERT ’98, 
v. WınıwArTer ’00, Arten ’04 for reviews of the literature upon this 
subject among the mammals. 
Nussbaum ’80, was among the first to advance the view that the 
primitive sex-cells are derived directly from undifferentiated embryonic 
cells reserved exclusively for this destiny at an early stage of develop- 
ment. This view has been supported by Nusspaum ’80 and E1GenMANnNn 
’92 and ’96, in their work upon the Teleosts, by Woops ’03 and Brarp | 
700 and ’04 upon the Elasmobranchs, Wuenuer ’99 for Petromyzon, by 
Nusspaum ’80 upon the Amphibia, and by Horrmann ’92 and Nuss- 
BAUM ’O! upon the Birds. 
According to another view, they arise by the transformation of 
peritoneal cells. This is the view held by the great majority of writers 
upon this subject, not only of the earlier ones, but of the more recent 
as well. Many authors, like Braun ’77, Benpa ’89, NAceEL ’89, ALLEN 
'04, ete., considered the spermatogonia to arise from peritoneal cells or 
their derivatives during embryonic stages alone, after which time, in- 
crease in their number would be brought about by cell division alone, 
not by continued transformation of peritoneal cells. On the other hand, 
Bausıanı ’79, Prenant ’87, Loser ’99—’02, ScHönrsLp 701 held that 
all, or at least a large part, of the functional spermatogonia arise by 
transformation of the peritoneal derivatives during post-embryonie and 
adult life. Rreaup ’99 and ’01, who originally held the view that the 
spermatogonia of the Mammals are renewed during adult life by pro- 
liferation from the Sertolian cells, has more recently expressed the 
opinion that they arise during early stages between birth and sexual 
maturity, from peritoneal derivatives-vegetative cells, but not as he had 
formerly supposed-during adult life. He gives a thorough review of the 
literature upon this subject. 
Braun ’77, Baupıanı ’79, and more recently Comrr ’98, v. Wını- 
WARTER 00, and SKROBANSKY ’03, have all similarly held that the oögonia 
arise by differentiation from peritoneal cells during embryonic life. The 
literature upon this subject is well reviewed by v. WınıwARrTeR ’00 and 
Corrt.’98. Every one who has hitherto studied this problem in the 
Reptiles and Mammals has come to similar conclusions, namely, that 
the sex-cells arise from peritoneal cells. 
In a recent paper, ’04, I have myself advocated this same view, 
arrived at incidental to my studies upon the embryonic development of 
the ovary and testis of the pig and rabbit. I distinetly admitted, how- 
ever, the possibility that certain peritoneal cells were destined, because 
of unobserved differences, to develop at various periods of embryonic 
and post-embryonic life into oögonia or spermatogonia. At the same 
time, it seemed no less plausible to consider the peritoneum to be 
