GERM-CELL HISTORY IN THE BROOK LAMPREY 115 



GENERAL SUMMARY OF OBSERVATIONS 

 A. Origin and early history of the germ cells 



1. The germ cells are first recognizable in the American brook 

 lamprey when the mesoderm separates from the entoderm, as 

 large yolk-laden cells which become included in the mesoderm. 

 Their history previous to this time could not be traced. Their 

 large size, however, indicates that they are early segregated cells. 



2. The number of germ cells that become included in the meso- 

 derm is small. There is evidence that many of them never 

 reach the germ-gland region. Some of these degenerate before 

 dividing, others form cysts in other regions of the body, and the 

 possibility is suggested that some of them may be extruded into 

 the lumen of the intestine in early stages. 



3. During the early period of their history the germ cells 

 shift from a lateral position in the mesoderm to a median posi- 

 tion. The change in position is accredited to a shifting of the 

 tissues surrounding the cells and, to a lesser extent, to independ- 

 ent migration. 



4. The germ cells begin to lose their yolk when the larva is 

 about 5.5 mm. long, and no yolk remains in the cells when the 

 larva is 10 mm. long. They do not begin to divide until the 

 larva is about 20 mm. long. 



5. The germ cells may be distinguished from the somatic 

 cells by their size, structure, and location. 



B. Period of secondary division 



6. When the larva is about 20 mm. long the germ cells begin 

 to divide by mitosis. 



7. After each mitosis the germ cells either separate or remain 

 together, forming cell nests. Peritoneal cells migrate in and 

 form follicles around the individual cells and cysts. 



8. An astrosphere is distinguishable in the germ cells of this 

 stage. 



9. A vitelline body is found in the cytoplasm. Its origin could 

 not be ascertained. It becomes a very prominent structure in 

 the growing oocyte of later stages. 



