GERM-CELL HISTORY IN THE BROOK LAMPREY 49 



cast out of the nucleus. The act of extrusion was not observed. 

 The body found by Dodds, however, has only a transitory exist- 

 ence in the cytoplasm, so it cannot correspond to the vitelline 

 body. Since the vitelline body is found to be a very prominent 

 structure in the oocyte of the lamprey, a full discussion of it 

 will be reserved for a future paper which will describe the 

 growth period of the egg. 



Witschi ('04) found that sex could be distinguished in frog 

 larvae before there was any differentiation of the germ cells, 

 by the fact that in the larvae destined to become females the 

 gerin cells remained along the periphery of the gland, while 

 in those destined to become males the cells migrated into the 

 interior of the gland in very early stages. No differences of this 

 kind have been found in the larvae of the lamprey during the 

 indifferent period. Neither are any genital cavities formed in 

 the gland by means of which the sexes may be distinguished pre- 

 vious to germ-cell differentiation, as is the case in amphibians. 



3. Summary of the period of secondary division. During the 

 period of secondary division (larvae 20 mm. to 35 mm. long) 

 the germ cells multiply by frequent mitoses. The resulting cells 

 may remain together after division, enclosed by a common 

 follicle, or they may become separated by the migration of 

 foUicle or peritoneal cells between them. The result is that in 

 all glands both cysts and isolated germ cells are found. The 

 germ cells are distinguished from the somatic elements of the 

 gland by their size, the structure of their cytoplasm, and the 

 form and structure of their nuclei. There is no indication that 

 germ cells are derived from somatic cells or somatic from germ 

 cells. Usually most of the cells of a cyst are found in one or 

 another phase of mitosis at the same time, but some of the cells 

 of the cyst may be in a resting stage while others are dividing. 

 In the cytoplasm of the germ cells a centrosphere with a centro- 

 some is often visible; also a vitelline body of unknown function 

 and origin. Spermatogonia and oogonia cannot be distinguished 

 during this period. Neither are there any other characters by 

 means of which future males and females may be distinguished. 



