MIGRATION OF SEX-CELLS OF FUNDULUS HETEROCLITUS. 339 



5. Migration is passive, being due to forces of growth which 

 are altogether external to the cells themselves. These forces of 

 growth are factors common to the development of the organs 

 formed in the body of the teleost embryo. 



6. Evidence derived from this study of Fundulus is an abso- 

 lute harmony with the theory of early segregation of these pri- 

 mary sex-cells. 



NOTE 



Some time after the manuscript of this paper had been sent 

 to the press, an extensive article by Okkelberg, entitled, " The 

 Early History of the Germ Cells in the Brook Lamprey, Ento- 

 sphcnus zi'ildcri (Gage), up to and Including the Period of Sex 

 Differentiation," appeared (Jour. Morph., Vol. 35, No. I, 1921). 

 This article contains much data and many important conclusions, 

 and it is to be noted (on pages 35 and 36) that the author, in 

 considering the sex cells, has discussed their methods of migra- 

 tion. It is of great interest that the conclusions reached by Ok- 

 kelberg on this matter for the lamprey are very similar to our 

 own upon Fnndnlus. 



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