No. 612] 



DEATH IN UTERO OF MOUSE 



741 



the 84 embryos from albino parents, only 2, or 2.3 per 

 cent., were degenerating. This makes it seem quite prob- 

 able that the homozygous yellow zygote develops for a 

 time and then dies. 



In our study no attempt has been made to investigate 

 the very early stages, as was done by Kirkham, but a 

 large number of embryos have been obtained from non- 

 suckling females pregnant from 13 to 19 days.^ In all 

 688 embryos have been examined. These have been ob- 

 tained from (1) yellow females mated to yellow males, 

 (2) yellow females mated to non-yellow males (choco- 

 lates), (3) non-yellow females (chocolates) mated to yel- 

 low males, and (4) non-yellow females mated to non- 

 yellow males. In this last mating most of the parents 

 were self blacks. 



During the investigation two distinct types of dead em- 

 bryos were encountered, (1) those in which development 

 had ceased shortly after implantation, corresponding to 

 those described by Kirkham, and (2) a few which seem 

 to have developed normally till about the thirteenth day 

 and then died, presumably because of overcrowding in 

 the uterus. These latter were characterized by theiir 

 dead, yellow appearance and smaller size as contrasted 

 with the living pink color and larger size of the normal 

 embryos. The first kind has been designated ''dead em- 

 bryos A" in the tables, while the second kind is classed 

 as "dead embryos B." We are primarily concerned with 

 "dead embryos A/' and it is to be understood that ref- 

 erence is to this type unless specifically stated otherwise.-^ 

 Similarly, by "living embryos" we mean those which 



