PORIFERA, CQALENTERATA, VERTEBRATA 101 
means a complete list but indicates the range of 
forms studied and the current interest in this subject. 
Some of the characteristics by means of which 
germ cells can be distinguished in vertebrate embryos 
are as follows: (1) the presence of yolk, (2) an 
ameboid shape, (3) large size, and (4) slight staining 
capacity. By sectioning embryos of various ages 
the changes in position of the germ cells can be fol- 
lowed with considerable accuracy. Most investi- 
gators agree that the movement of the germ cells from 
the tissues where first observed to the genital ridge 
is caused by ameboid activities of the cells themselves 
and by changes in the position of the organs of the 
embryo. The paths of migration of four verte- 
brates, a turtle, Chrysemys, a frog, Rana, the gar 
pike, Lepidosteus, and the fresh-water dogfish, Amia, 
are shown in Fig. 6. For example: 
‘In Lepidosteus the sex-cells [Fig. 6, 3, SJ] first 
seen in the ventral and lateral portions of the gut- 
entoderm [Int] migrate to occupy a position in the 
dorsal portion of it, from which they pass dorsally 
into the loose mesenchyme that forms the substance 
of the developing mesentery [Mes]. As the mesen- 
tery becomes more narrow and compact, owing to 
the increase in size of the body cavity, the sex cells 
migrate to its dorsal portion and laterally to 
the sex-gland anlagen (Fig. 6, 4, Sc). Roughly 
speaking, one-half of the total number of sex-cells 
reach the sex-gland anlagen, the remainder being 
distributed between the intestinal entoderm, the 
mesodermal layers of the intestine, the mesentery, 
