ORIGIN OF DEFINITIVE SEX-CELLS 



455 



In one embryo the number of primordial germ-cells was count- 

 ed and was found to be 743; in another embryo the number was 

 found to be 860. These numbers are only approximate but are 

 probably 90 per cent correct. In the first embryo there were 

 about twice as many on the left side as on the right, while in the 

 second embryo there were about five times more numerous on the 

 left side (fig. 3). The number of germ-cells in the other embryos 



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Fig. 3 Portion of a transverse section through indifferent gonads of a 4-day 

 chick embryo. The drawing is semi-diagrammatic, altliough the outlines, posi- 

 tion and number of the germ-cells are exact reproductions of conditions existing 

 in a single section of this embryo. Notice the greater ninnber of primordial 

 germ-cells in the left gonad. 



was not counted, but it could be easily seen that the number in 

 the left gonad was far in excess of that in the right gonad (fig. 3) . 

 This very suggestive fact was published by Firket ('14) for the 

 first time. It is of course proof positive that the germ-cells 

 migrate actively and are not distributed by tissue displacements 

 as was advocated by von Berenberg-Gossler ('12). This unequal 

 distribution of the germ-cells in favor of the left side may account 

 for the fact that the gonad of that side is, from its first appear- 

 ance, the more massive. 



