Artificial Parthenogenesis and Heredity 295 



But, while the rate of segmentation is thus essentially de- 

 termined by the egg, the same is not true for the rate of the 

 further development of the embryo. This can be shown in 

 comparing the time of appearance of certain characters in the 

 hybrids and pure breeds of two related species. Bancroft and 

 the writer measured the time which elapses from the moment 

 of fertilization to the time of appearance of the heart beat, the 

 circulation, the red chromatophores of the yolk, the black 

 chromatophores, etc., in the embryos of Fundulus heteroditus, 

 Fundulus majalis, and the two hybrid forms majalis ?X/ieiero- 

 ditus $, and heteroditus ^Xmajalis $. The rate of development 

 is much slower in the pure breed of majalis than in the pure 

 breed of heteroditus. If the egg of majalis is fertilized with 

 the sperm of heteroditus the rate of segmentation is practically 

 the same as if it were fertilized with the sperm of majalis. 

 The appearance of certain embryonic characters is, however, 

 accelerated as the following table shows. 



TABLE XXXIX 



It is obvious that the entrance of the heteroditus sperm into 

 a majalis egg causes the red and black yolk chromatophores 

 to appear from two to three days earlier than if the majalis 

 sperm enters the same egg. 



This may be accounted for by the fact that the process of 

 development of the embryo is essentially determined by the 

 transformation of yolk material into the living organs of the 



