Embryonic History of the Germ-Cells of the Loggerhead Turtle. 337 



It remains to discuss the mode of the extravascular migration — i. e., 

 whether active (amoeboid) or passive, or both. A passive migration is 

 strongly urged by von Berenberg-Gossler. He thinks the germ-cells 

 are passed from the lateral to the medial entoderm, and from the lateral 

 visceral mesoderm to the mesentery, and then across the coelomic angle 

 by the mechanical process of unequal growth. Growth is no doubt a 

 factor in the shifting. But the chief factors inhere in the cells them- 

 selves, which (judged from their shape and the manner in which the 

 mesenchymal cells are compressed in the direction of a path towards 

 the root of the mesentery) is an active amoeboid movement of the cells. 

 The cells e\ddently move medially both within the endoderm and in the 

 mesoderm. Such as lie medially in the entoderm become inclosed 

 within the hind-gut, whence the majority later migrate into the mesen- 

 tery. 



Only two germ-cells were found at any stage within any blood- 

 channels. But no confusion between blood-cells (hemoblasts) and 

 germ-cells seems possible. The germ-cells are at all stages larger than 

 the largest granulocyte (eosinophil, whose granules also stain black 

 in iron-hematoxylin) and the largest hemoblast, the character and 

 staining capacity of whose nucleus is very different. There is therefore 

 no room for doubt that in chick (Swift) and duck (von Berenberg- 

 Gossler) the germ-cells actually do migrate (passively) towards the geni- 

 tal ridge largely through the blood-vessels. The locus of original segre- 

 gation in birds is such as to provide in the blood-channels the most 

 favorable method for the longer portion of the route of migration to 

 the sexual gland. 



As Allen has already suggested, the reason for the mesodermal origin 

 of the germ-cells in urodeles and possibly certain other vertebrates may 

 be a relatively early separation of mesoderm from entoderm — that is, 

 the germ-cells of the primitive entoderm may have become separated 

 with the portion which contributed to the splanchnic layer of the 

 mesoderm before they could migrate from the entoderm proper. 



In Lacerta von Berenberg-Gossler describes cells comparable to the 

 so-called primordial germ-cells passing from the entoderm to both 

 layers of the mesoderm, and to the dorsal layer of the intermediate 

 cell-mass, where caudally the cells are said to contribute largely to the 

 formation of the Wolffian duct. Von Berenberg-Gossler interprets his 

 results to mean a belated origin of mesoderm from entoderm. He very 

 ingeniously suggests that these "germ-cells" wander also to the sex- 

 gland, where they may transform into mesenchymal-cells, and then 

 retransform into germ-cells, but presents no histologic data in support. 

 Von Berenberg-Gossler's results and his interpretation are unique and 

 do not seem to be capable of being brought into harmony with the 

 results obtained in any other form thus far studied, nor with any 

 common interpretation. Von Berenberg-Gossler's interpretation is 



