ON THE PRECOCIOUS SEGREGATION OF THE 



SEX-CELLS IN MICROMETRUS AGGREGATUS, 



GIBBONS. 1 



CARL H. EIGENMANN. 



While preparing the sections for the ontogeny of Microme- 

 trus aggregatus, one of the viviparous Holconotidae, I frequently 

 observed large, indifferent cells in the mesoblast. I at first sup- 

 posed them to be cells in a pathological condition. When, how- 

 ever, all the eggs from one ovary were observed to contain such 

 cells I re-examined every embryo, and soon found that the cells 

 are not pathological, but are a normal structure present in all 

 embryos of a certain age. Further study showed them to be 

 sex-cells of the future germinal epithelium. Since our knowl- 

 edge of the early stages of the sex-cells of vertebrates does not 

 extend back beyond the condition described by Balfour for 

 Elasmobranchs, and since I have been able to trace them back 

 to probably the fifth segmentation, I have thought best to give 

 their history from their appearance, or rather, from the time 

 when they become distinguishable from the surrounding cells 

 till they are lodged in the mesentery of the hind gut, where Bal- 

 four has described them for Elasmobranchs. 



Our knowledge of the sex-cells in general has been summed up 

 by Weismann as follows : " In certain insects the development 

 of the egg into the embryo, that is, the segmentation of the egg, 

 begins with the separation of a few small cells from the main body 

 of the egg. These are the reproductive cells, and at a later period 

 they are taken into the interior of the animal and form its re- 

 productive organs. Again, in certain smaller fresh-water Crus- 

 tacea (Daphnidae) the future reproductive cells become distinct 



1 It gives me great pleasure to acknowledge my indebtedness to Dr. Silas M. 

 Mouser, of San Francisco, California, who extended to me the use of his perfectly 

 equipped bacteriological laboratory, without which courtesy my work would have 

 been materially retarded. 



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