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 heteroclitus, 
Fundulus majalis, and the two hybrid forms majalis 2X hetero- 
clitus 2, and heteroclitus 2X majalis ¢. The rate of development 
is much slower in the pure breed of majalis than in the pure 
breed of heteroclitus. If the egg of majalis is fertilized with 
the sperm of heteroclitus the rate of segmentation is practically 
the same as if it were fertilized with the sperm of mayjalis. 
The appearance of certain embryonic characters is, however, 
accelerated as the following table shows. 
TABLE XXXIX 
BEGINNING OF First APPEARANCE OF 
BREED ; Z 
Heart “| Cireula-.| Grecmas | Chrome | mont of, 
Beat tion tophores | tophores Eyes 
Heteroclitus 2 X Heteroclitusé .|4d. 2h.j4d.15h./3d. 22h.)3d. 16h./7d. 6h. 
Heteroclitus2 X Majaliss ....\4d.17hj5d. 12h.j4d. 17h.j4d. 3h./8d. 2h. 
Majalis 2 X Heteroclitusé ....)5d. 14h.6d. 14h.6d. 8h.j5d. 18h.9d. 20h. 
Majalis? X Majalisé........ 5d. 19h.j6d. 18h./9d. 10h.)7d. 17h./9d. 14h. 
It is obvious that the entrance of the heteroclitus 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 
