116 PETER OKKELBERG 



10. Numerous mitochondria are present in the cytoplasm of the 

 germ cells in most phases of their history. 



11. Two plasmosomes are present in the primordial germ cells 

 but in the growing oocyte there is only one. 



12. The period of secondary division lasts until the larva is 

 about 35 mm. long. During this period the larva appears indif- 

 ferent as to sex. 



C. Period of sex differentiation 



13. The period of sex differentiation extends from the time 

 the larvae are about 35 mm. in length until they are about 70 

 mm. long. In some larvae, however, sex differentiates much 

 earlier than in others. During this period the sex of the larvae 

 is indeterminate. The condition may be described as juvenile 

 hermaphroditism. 



14. A varying number of oocytes appear in practically all the 

 glands during this period, so that a series of glands might be 

 arranged possessing from per cent to 100 per cent of oocytes. 



15. The changes taking place in the oocytes during the sjmap- 

 sis phase are described. 



16. Numerous germ cells degenerate during this period. De- 

 generation may take place during the synapsis phase, the growth 

 phase, or the indifferent phase of the germ-cell history. 



17. When sex is established the germ cells belonging to the 

 opposite sex disappear or remain in the gland in a rudimentary 

 condition. 



CONCLUSIONS 



The following general conclusions may be drawn from the 

 above study of the germ cells of the American brook lamprey: 



1. The germ cells are segregated very early in the life of the 

 animal even before the germ layers are definitely established. 

 They are first recognizable when the mesoderm separates from 

 the entoderm. 



2. The definitive germ cells take their origin from no other 

 source than the primordial germ cells and the germ cells take no 



