PORIFERA, CCELENTERATA, 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, SI] 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 [Jfes]. 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, 



