No. 3.] SEGREGATION OF THE SEX-CELLS. 49 1 



number in early stages when the cells are quite large. The 

 reduction in size can, therefore, be explained only by supposing 

 that the individual cells are reduced in size during develop- 

 ment. It would be interesting to consider here the causes that 

 lead these sex-cells to again grow and divide. Since, however, 

 this process does not begin in the stages under consideration, 

 this matter must be left till later stages are examined. Another 

 question seems more pertinent. In how far do the sex-cells, in 

 the ontogeny of Micrometrits, repeat the phylogenetic history 

 of these elements in vertebrates ? I am not aware that the 

 sex elements have been traced nearly as far back in any other 

 vertebrate, and to explain the origin of the sex-cells in verte- 

 brates in general from a type the development of which differs 

 so much from related species is perhaps rash. Still it is not 

 evident how causes which have brought about the changes in 

 the early history of Micronietrns should affect the sex-cells. In 

 Branchiostovia the germinal region is much more extensive than 

 in the Craniata. It is true that the ovaries and testes extend 

 well forward in many fishes during the breeding season, but in 

 embryos of several species I have examined, the "anlage " of the 

 sexual organs is confined to a much more restricted region in 

 the posterior part of the body cavity, and its anterior extension 

 is a later, secondary feature. Micrometrus would indicate that 

 the present position of the sexual glands in Craniata is not the 

 primitive one, but that the anterior glands in Branchiostoma 

 probably represent the earlier condition. From this position 

 the germinal region has been extended backward, the anterior 

 part undergoing atrophy still later. 



I have not been able to examine the special literature bear- 

 ing on this subject, since the observations were completed in 

 California, at a distance from modern scientific libraries. For 

 this reason I shall be pardoned, if, perchance, I have not given 

 due credit to the works of predecessors. 



Indiana University, Bloomington, Indiana, 

 September 17, 1891. 



