GERM-CELL HISTORY IN THE BROOK LAMPREY 41 



movement of the cells. The original double anlage of the germ 

 gland is reduced to a single median gland in later stages, due to a 

 fusion of the two parts along the midline. This fusion begins 

 at the cranial end of the gland. The germ cells which do not 

 reach the germ gland probably degenerate. Some of them form 

 extraregional cell nests, the fate of which is not known. There 

 is no division of the germ cells before the larvae have reached a 

 length of about 2 cm. Long before this all the yolk has been 

 absorbed. Granules staining like chromatin are found in the 

 cytoplasm around the nucleus when the yolk begins to disappear 

 in the cells. When the yolk has been absorbed, two cytoplasmic 

 bodies are found, one a spindle-shaped, granular vitelline body, 

 the other an astrosphere (yolk nucleus). The origin of the vitel- 

 line body was not ascertained. The number of the primordial 

 germ cells is small. In one count thirty-six were found. There 

 is no indication during the early stages that the germ cells are 

 derived from any other source than the early segregated cells. 



C. Period of secondary division 



After the primary period of rest, already described, during 

 which the germ cells do not divide, they enter upon a period of 

 division (table 2). In the following pages the changes are de- 

 scribed which take place in the germ gland during this period, 

 which begins when the larva is about 20 mm. long. 



1. Observations on Entosphenus wilderi. a. Larva 2L5 mm. 

 long, August 23 (fig. 34). In this larva, somewhat more than 

 four months old, the germ-gland fold is still interrupted. While 

 many single germ cells are found and many cysts of less than 

 eight cells, there are a few of from eight to sixteen cells, and the 

 latter mmiber is rarely exceeded in this stage. Most of the germ 

 cells are in the resting stage, but a few are in various phases of 

 mitosis. 



The gland is 2.5 mm. long and extends from a point near the 

 middle of the mesonephros, about 2.5 mm. back of the cranial 

 end of the coelom, to a point about 2 mm. in front of the anal 

 opening. Its position, therefore, coincides about with the 



