GERM-CELL HISTORY IN THE BROOK LAMPREY 37 



A study of the methods by means of which germ cells migrate 

 from the entoderm into the mesoderm in the various groups of 

 vertebrates, and of the time at which this migration takes 

 place, shows that these groups may be arranged in an interesting 

 phylogenetic series as represented in the following ^diagrams 

 (text figures A, B, C, D). 



In the lamprey (text figure A) the germ cells are shown as being 

 included in the mesoderm at the time when it becomes separated 

 from the entoderm. In Triton (text figure B), according to 

 Abramowicz ('13), the germ cells migrate into the mesoderm be- 

 fore the dorsal mesentery is formed, but much later than in the 

 case of the lamprey. In the frog, according to Allen and others, 

 the germ cells are separated from the yolk entoderm at the 

 time the dorsal mesentery is formed, as shown in text figure C. 

 In each of the first tw^o forms, therefore, the original germ-cell 

 anlage is double, while in the latter it appears to be unpaired at 

 first and to lie along the middorsal line above the gut entoderm. 

 In the lamprey, although the original anlage is paired, the germ 

 gland later becomes single by a fusion of the two parts on the 

 ventral side of the dorsal aorta. In Triton the paired anlagen 

 fuse, but later they become paired again, and in the frog the 

 original unpaired anlage becomes paired. In reptiles and mam- 

 mals the germ cells usually separate from the entoderm much 

 later than in the other three forms, often after they have lost all 

 their yolk. Then migration follows: first into the splanchnic 

 mesoderm, then through the dorsal mesentery, and from there 

 to the germ-gland anlage on each side (text figure D). 



c. Relation of germ cells to body cells. In the later embryonic 

 stages of many forms, when the germ cells have j-eached their 

 final destination in the region of the germ gland, they lie among 

 the peritoneal cells covering the definitive germ-gland anlage and 

 apparently form a part of the peritoneal membrane. This fact 

 gave rise to the idea that the cells were derived from the peri- 

 toneal cells, so that this portion of the peritoneum became known 

 as the germinal epithelium. In the lamprey there is, strictly 

 speaking, no germinal epithehum since the germ cells never form 

 a part of the peritoneal membrane, but are independent elements 



