Embryonic History of the Germ-Cells of the Loggerhead Turtle. 335 



My second surprise was to find practically no germ-cells in any 

 vascular channels; only two exceptions were noted. Nor does there 

 seem to be any possibility of confusion of sex-cells with blood-cells, 

 judging from Swift's figures or on the basis of my sections of the logger- 

 head-turtle embryo. Swift's observations have now been confirmed 

 by von Berenberg-Gossler for chick and duck. Von Berenberg-Gossler 

 was unable, however, to find similar conditions in the case of the lizard 

 embryo. The condition of abundant intravascular germ-cells in the 

 chick embryo and the duck embryo is so far unique. Conditions 

 respecting the germ-cell history in birds are evidently different from 

 those obtaining in reptiles and certain other amniotes, and depend 

 upon the cephalic (pro-amniotic) site of origin of the germ-cells and 

 their close relation to the vascularizing mesoderm. 



Considering the initial very close relationship (both spatial and 

 genetic) between the entoderm and the mesoderm at the time when the 

 latter is in the early stages of vascularization and the considerable 

 migratory capacity of the primordial germ-cells, the presence of these 

 cells outside of the usual migration route, even in the blood-vessels, 

 is nothing extraordinary. In the chick the initial source of origin is 

 in the entoderm at the anterior extremity of the blastoderm in a region 

 originally free of mesoderm. The cells begin their first migration about 

 the time the vascularizing mesoderm invades this region, and so readily 

 enter into the blood-vessels by which they are transported in large part 

 caudally to the medial splanchnopleure, whence they pass via the 

 mesentery to the gonads (Swift). In forms like the turtle and the 

 dog-fish, where the germ-cells are originally scattered caudally in the 

 area opaca, they do not in large numbers come into so intimate early 

 relation with the young blood-vessels, and in consequence reach the 

 medial splanchnopleure largely via the visceral layer of the mesoderm 

 by their own amceboid activity, meanwhile passing through a tempo- 

 rary stage of sharp segregation into the paired cords of the area pellu- 

 cida. But even here it is apparently an easy matter for the germ-cells 

 to become involved with the blood-channels either through active 

 migration or passive inclusion. There is nothing, therefore, inherently 

 contradictory or unique in the condition of the germ-cell migration as 

 it obtains in the chick and duck. The difference is one of degree 

 rather than kind, and depends upon the difference in initial location 

 of the primordial germ-cells with respect to the mesoderm and the head 

 end of the embryo. 



The very careful work of Allen, in which he counted the number of 

 germ-cells at the different stages of development in different areas in 

 Chrysemys, clearly shows that the total number of germ-cells in different 

 individuals of a species varies within wide limits (302 to 1,744), and 

 that the time and rate of migration also vary considerably. I there- 

 fore made no attempt to approach the question of the route and manner 



