GERM-CELL HISTORY IN THE BROOK LAMPREY 39 



situated in the mesenchyme dorsal to the peritoneum. Some- 

 times the germ cells may lie so pressed against the peritoneum 

 that its cells become greatly flattened, but the continuity of the 

 peritoneal epithelium is apparently never broken. I have found 

 no evidence that any germ cells are formed out of peritoneal 

 cells at any stage. 



d. Number of germ cells. No evidence was found that divi- 

 sion takes place in the germ cells of the lamprey before the larva is 

 about 20 mm. long. The germ cells are few in number, and, as 

 the young larva continues to grow, they become more and more 

 scattered along the whole length of the body cavity, so that in 

 following a series of sections they are often found far apart. In 

 a larva about 8 mm. long, in which the germ cells stood out 

 with great clearness, thirty-six were counted. These were dis- 

 tributed through 153 10-m sections or along 1.53 mm. of the body 

 length, as shown in figure 10. The distribution of the cells by 

 sections was as follows (table, p. 40) : 



It will be seen that at this time no germ cells are found in the 

 caudal region of the body for about 0.58 mm. in front of the 

 anal opening, and cranially they do not extend any farther 

 than to about the middle of the coelomic cavity. 



The number of germ cells found in this larva comes very near 

 that found by Beard ('02) in larvae of Petromyzon planeri, 

 but it can hardl}^ be cited as supporting his theory, especially 

 since it is known that many of the germ cells never reach the 

 germ-gland region. It is highly doubtful if the number of germ 

 cells segregated in the development of the embryo is constant, and 

 it certainly seems probable that the number of germ cells that 

 reach the germ gland must vary greatly in different individuals. 

 It would be futile, therefore, to form any conclusions concerning 

 the number of primordial germ cells from a count of those that 

 reach the germ gland. 



4. Conclusions and summary. Evidence presented supports the 

 theory that germ cells are segregated very early in the develop- 

 ment of Entosphenus wilderi. These primordial germ cells are- 

 first found in the yolk entoderm and become included in the 

 mesoderm when it separates from the entoderm. The germ cells 



