PORIFERA, COELENTERATA, VERTEBRATA 99 
not strange that he should have come to the con- 
clusion that the primordial ova arise from the 
epithelial cells of the genital ridge among which they 
were observed. Although this theory was accepted - 
by most embryologists, it has gradually been aban- 
doned until now it has very few supporters. 
The gonotome theory resulted from the studies of 
Riickert (1888) and Van Wijhe (1889). The germ 
cells appeared to these investigators to arise in a 
part of the segmental mesoblast of the embryo to 
which the latter applied the term ‘gonotome.’ 
From the gonotome they become embedded in the 
peritoneum. Thus the same cells are recognized as 
germ cells by the adherents of both theories, but a 
difference exists regarding their origin. 
The theory of early segregation has become the 
most prevalent view.of the origin of the germ cells 
of vertebrates, although there are many who still 
hold one of the other hypotheses. According to 
this theory the germ cells are set aside during the 
early embryonic stages before definite germ layers 
are formed, and they later arrive at the germinal 
ridge either by their own migration or by changes in 
the position of the tissues during development. 
The germinal epithelium theories have little if any 
evidence in their favor, since no one has actually ob- 
served a transformation of peritoneal or mesoblast 
cells into germ cells. On the other hand, there is an 
abundance of proof that these cells migrate from 
some distance into the position of the sex glands. 
According to Dustin (1907), Firket (1914) and 
