124 THE EGGS OF MAMMALS 



metrium thus preventing their injury. The fact that blasto- 

 cysts in culture also show unusual sensitivity leaves the 

 first two of these alternatives. 



The behavior and differentiation of the blastodermic ves- 

 icle at the time of implantation have been the object of 

 extensive investigation by mammalian embryologists since 

 the publication of Bischoff's (1852) classical memoir on the 

 subject. These investigations have been concerned chiefly 

 with presenting exact descriptions of the mode of implanta- 

 tion in various classes of mammals (see Robinson, 1904; 

 Grosser, 1909; Bonnet, 1903; Spee, 1915; Wilson, 1928; 

 Sansom and Hill, 1930) and the accompanying differentia- 

 tion of the vesicle. The physiological processes underlying 

 these phenomena have been scarcely investigated. 



The writer has been interested in the phenomenon of 

 the delayed pregnancy which seems to offer an opportunity 

 to exploit the processes occurring at implantation. Delayed 

 pregnancy, or late parturition, occurs notably in the lactating 

 mouse or rat which is carrying a set of fertilized eggs during 

 lactation. This is a result of the fact that mice and rats 

 have an oestrus period within 48 hours of parturition in 

 which normal mating and fertilization take place. Enzmann, 

 Saphir and Pincus (1932) have analyzed all the available 

 data in the literature and found that in mice and rats each 

 suckling young on the average prolonged pregnancy by 

 about 21 hours (see Figure 33), though this time of prolonga- 

 tion seemed to vary somewhat from strain to strain. An 

 examination of mated mice in a series of timed matings 

 disclosed the fact that the preimplantation vesicle in suckling 

 females failed to implant at the normal time but some time 

 later depending upon the number of young suckling (see 

 Kirkham, 1916, 1918). Once implantation occurs the growth 

 of the embryo proceeds at the rate characteristic of normal 

 embryos (Enzmann, 1935). Obviously the lactation process 

 results in the establishment of conditions in uUro which 

 inhibit implantation, and the rather exact relationship be- 

 tween the degree of delay of pregnancy and the number of 



