130 WILLIAM B, KIRKHAM 



attacked before any other members of the sets had started to 

 implant. 



Swift as is the destruction of abnormal blastulas after they 

 reach the uterus, it is nevertheless in yellow mice not so rapid 

 as to prevent them from secreting the substance which causes 

 the swelling and proliferation of the uterine mucosa, for this 

 process takes place in a normal manner, the epithelium dis- 



Fig. 1 Abnormal blastula from a yellow mouse, six days after ovulation (set 

 Y 45). This set of embryos consists of five normal and three abnormal blastulas; 

 two of the latter are illustrated in this and the following figures, and the third 

 is similar to this one. The normal set mates of this specimen are in an early 

 implantation stage, and while the uterine epithelium near this abnormal individ- 

 ual is still intact, the neighboring mucosa is much swollen. The section drawn 

 passes through the middle of the blastula, but the cavity is small and so located 

 with respect to the plane of sectioning that it is merely indicated by the lighter, 

 central area. Three invading phagocytes are shown (Pc), and several others ap- 

 pear in adjoining sections. X 600. 



appears, the lumen of the uterus is closed, and all this goes on 

 even though the blastula itself is being rapidly dissolved. This 

 is in marked contrast to the fate, mentioned above, of abnor- 

 mal blastulas in healthy non-yellow mice, which perish without 

 effecting the uterine reaction. However, the same phenomenon 

 of a normal, fully prepared implantation site with no trace of 

 embryonic cells likew ise occurs in about 50 per cent of the preg- 

 nant white females examined, which had either produced still- 



