223 
by which it is accomplished, the fact of this migration is clearly 
established by an extensive series of stages showing every step in the 
process. 
This is no new discovery, since BALFOUR (78) gave strong evidence 
- to show that in the Elasmobranchs, sex-cells migrate into the sex 
gland anlage from the base of the mesentery and from other positions 
in the peritoneum some distance removed from it. Brarp (0Q) and 
Woops (03) have given a much more complete account of this pro- 
cess. According to BEARD (00), “The germ path is a very definite 
one. It is from the yolk sac upwards between splanchopleure and gut 
in the hinder portion of the blastoderm” — this in Raja. He does 
not find the germ path to be so definite in Pristiurus. Woops (03) 
gives a very interesting account of the mi- 
gration of these cells in Acanthias. According fx > 
to him, they are transferred from the rim of Ry 3 
the blastoderm inwards along the splanchnic 3% (8: 
layer of mesoderm to the sex gland anlage. He Mes. yon 
considers it possible that a large part of this = Ss 
migration may be only apparent, being due a 
to the shifting of the tissues as a whole; but 
he is very certain that the movement of the 
sex-cells up through the mesentery is a true 
Fig. 5. Detail of region indicated in Fig. 2. X 721. 
Mes. Mesoderm not yet clearly split into somatopleure and 
splanchnopleure. End. Endoderm. Sea-C. Sex-Cell. 
migration. EIGENMANN (’92 and 96) records a migration of the sex-cells 
from before backward in Micrometrus and Cymatogaster. WHEELER (’99) 
found them to appear in Petromyzon in the hypoblast before the 
mesoderm is split off from it. When the division does take place, they 
are incorporated into the splanchnic mesoderm. They later shift from 
this lateral position to the median sex gland anlage. NussBaum (’80) 
observed them in the early stages of Rana and assumed from their 
appearance that they had migrated from the vitellus. Bourn (00) 
found them at the root of the mesentery in Rana, but, although he 
noted their resemblance to the endoderm cells, he claimed to find 
intermediate forms joining them with the mesoderm cells from which 
he considered them to have been derived. NussBAuM (01) found the 
sex-cells in the splanchnic mesoderm of the chick some distance from 
the sex gland anlage. He assumed that they arise at the edge of the 
blastoderm and migrate inward. Horrmann (92) found them in 
Haematopus, Sterna, and Gallinula during early stages, where they 
