34 PETER OKKELBERG 



My observations and those of Wheeler ('99) show that the germ 

 cells appear first in the posterior region of the body, probably 

 in a small area around the blastopore. None have been seen to 

 separate from the entoderm very far craniad of this region, so 

 that those found later in the region further craniad gain this 

 position by some form of migration. In Entosphenus wilderi the 

 peristomal and the paraxial regions of the mesoderm are con- 

 tinuous and do not differ in structure or in origin as maintained 

 by Hatta ('92, '07). The only distinction found between them 

 is that the mesoblast of the paraxial region is delaminated earlier 

 than that of the peristomal region, but only the peristomal 

 mesoderm carries the germ cells in early stages. In Entosphenus 

 wilderi no cells corresponding to Kupffer's teloblast cells were 

 found in this region. These may have been either germ cells or, 

 as he supposed, mesoderm cells. 



The condition in the lamprey is in favor of the theory of early 

 segregation. The large j^olk-laden cells that are at first found 

 among the other yolk-bearing cells of the entoderm in the 

 caudal region of the body become included in the peristomal 

 mesoderm when it separates from the entoderm by a process of 

 delamination. These cells retain their embryonic structure for 

 a long tmie after the other elements of the mesoderm have become 

 differentiated. Even after all their yolk has been used up, 

 they remain as large conspicuous cells among the smaller somatic 

 cells of the germ gland. Up to this point the history of these 

 cells has been traced and the later history to a time when they 

 begin to divide. Later it will be seeji that there is in Ento- 

 sphenus wilderi no evidence that any other cells take part in the 

 formation of definitive germ cells. What evidence there is in 

 other forms will now be considered. 



Two lines of evidence have been advanced for the secondary 

 origin of germ cells from mesodermal elements. The first is that 

 transitional stages have been found between mesodermal cells 

 and true germ cells (Abramowicz, Dustin, Firket, Gatenby, and 

 others), but in most cases the figures which purport to repre- 

 sent this transition are not convincing. The second line of 

 evidence is that advanced by Kuschakewitsch ('10). He found 



