THE FATE OF HOMOZYGOUS YELLOW MICE 127 



and proliferation of the cells of the uterine connective tissue, 

 followed shortly by the dissolution of both these cells and those 

 of the overlying epitheUum, thus supplying food for the embryos 

 and effecting a closer union between the embryonic and maternal 

 tissues. 



It is still unknown why in large sets of blastulas not all be- 

 come implanted, but the writer has seen no evidence in 

 white mice of morphological defects in any members of such sets, 

 prior to implantation, and it more likely depends on either physio- 

 logical deficiencies present in individual cases, on a time factor, 

 or on these two factors acting together. All the blastulas of a 

 set do not reach the uterus simultaneously, nor do they all im- 

 plant at the same moment, and it is conceivable that the con- 

 ditions are such that as the separate blastulas implant, the uterus 

 offers an increasing resistance to those remaining free. What- 

 ever the cause, it is at least certain that in normal mice, other 

 than yellow, the blastulas that are unable to implant fail to 

 arouse the specific response from the uterine wall, soon become 

 moribund and are phagocytized. 



Once implanted, mouse embryos of all colors appear well able 

 to meet the demands of development up to the time of par- 

 turition, and the few instances of dead implanted embryos seen 

 by the writer and those designated 'B' embryos by Ibsen and 

 Steigleder ('17) are almost certainly due to pathogenic conditions 

 in the maternal tissues. When such conditions are extreme, an 

 entire litter may die in utero, while less critically adverse en- 

 vironments will cause the death of only the less resistant em- 

 bryos, for before, as well as after, birth they are individualized 

 organisms. 



At the time of birth the second crisis occurs, and feeble mouse 

 embryos of all colors which have succeeded in reaching this cru- 

 cial stage in their lives succumb to the difficulties inherent in the 

 transition from a parasitic to an independent existence. 



