ACCOUNT OF THE GERM-CELL CYCLE 33 



trated by the case of Amia and Lepidosteus. While 

 in the forms that I have studied they are first to be 

 observed in the entoderm, I am quite open to convic- 

 tion that in other forms they may migrate from tliis 

 layer into the potential mesoderm before the two 

 layers are separated, as shown by Wheeler in Petro- 

 myzon.'^ 



Swift (1914) has recently obtained evidence which 

 seems to prove that not only do the germ cells of 

 the chick migrate by ameboid movements but they 

 enter the blood vessels and are distributed by the 

 blood stream to all parts of the embryo and vascular 



area. 



The migration of the germ cells has been noted in 

 many invertebrates and has been fully described 

 in chrysomelid beetles (Hegner, 1909a). In these 

 insects the primordial germ cells are segregated at 

 the posterior end of the egg at the time the blasto- 

 derm is formed (Fig. 36, C). The blastoderm is 

 never completed just beneath them, but a canal, 

 called the pole-cell canal, remains. Through this at 

 a later embryonic stage the germ cells migrate by 

 means of ameboid movements. 



"As soon as the germ cells of CalUgrapha have 

 passed through the pole-cell canal, they lose their 

 pronounced pseudopodia-like processes and bocuine 

 nearly spherical (Fig. 37, E) ; nevertheless, they 

 undergo a decided change in position. They move 

 away from the inner end of the pole-cell canal, and 

 creep along between the yolk and the germ-band. 

 Thus two groups are formed near the developing 



D 



