336 A. RICHARDS AND JAMES T. THOMPSON. 



(peripheral endoderm, Allen). The complete germinal path 

 from this position to one lateral to the hind gut may be followed 

 in almost any embryo of from 46 to 50 hours. This very advan- 

 tageous condition is made possible by the greater development 

 near the middle of the embryo, for it is only a natural result of 

 embryo formation by concrescence that development is progres- 

 sively greater anteriorly from the point of convergence of the 

 germ ring. 



These cells are transported from the edge of the embryonic 

 region medially, to positions just beneath or within the endoder- 

 mal cell mass, as the case may be. They are carried passively 

 from one position to another by the same forces of growth which 

 bring together the halves of the germ ring. The influence of 

 this factor can scarcely be over emphasized. Although not out- 

 wardly as apparent as in earlier stages, these forces are neverthe- 

 less responsible for the flowing of the streams of embryonic 

 material towards the future position of the organs which are to 

 develop therefrom. 



The sex-cells come to lie within these shifting layers of em- 

 bryonic endoderm and mesoderm and naturally accompany these 

 layers in their changes of position. Because of the fact that the 

 movement of the sex-cells is not active but rather dependent 

 upon that of surrounding layers, the expression " migration " 

 seems rather unfortunate. Some term such as " translocation " 

 would perhaps be more truly expressive of the actual conditions. 



These cells come to lie in the edge of the embryonic region, 

 and when a portion of the undifferentiated cell mass gives rise to 

 gut endoderm and another to lateral mesoderm, they follow one 

 layer or the other. The sex-cells follow one or the other of these 

 layers until they reach a position lateral to the newly formed gut. 

 Which layer is chosen apparently depends upon chance. Those 

 cells which have been carried in the edge of the endoderm never 

 enter the gut, but move dorsally from the side of it into the lat- 

 eral mesoderm. Here they join the sex-cells which have been 

 carried in the mesoderm. By this time the split, resulting in the 

 formation of the ccelome between the splanchnic and somatic 

 mesoderm has taken place. Although the sex-cells are asso- 

 ciated with all parts of the lateral mesoderm before the forma- 



