The Corpus Luteum of the Guinea Pig 81 



suggest that luteal growth is stimulated at the time of implant- 

 ation of the blastocyst. This is shown by many species which 

 exhibit the phenomenon of delayed implantation, for example 

 many species of the Mustelidae (Wright, 1942) and the 

 Pinnipedia (Harrison, Harrison Matthews and Roberts 1952; 

 Rand 1955), and in species in which implantation is delayed 

 by concurrent lactation, for instance the bank vole (Cleth- 

 riomys glareolus) examined by Brambell and Rowlands (1936). 

 In all these there is evidence that the growth rate of the corpus 

 luteum slows down or may even stop when the blastocyst is in 

 the free-living state in the uterus, but when attachment takes 

 place luteal growth is resumed. Experimental evidence of the 

 role of the uterus in prolonging luteal activity has been pro- 

 vided by Nalbandov, Moore and Norton (1955), who have 

 shown that distension of the uterus of sheep by insertion of a 

 bead 8 mm. in diameter delayed the onset of the next oestrus 

 by about eight days. In the sheep, however, no difference in 

 size exists between the corpus luteum of the cycle and that in 

 the pregnant animal, so that it is not possible to show whether 

 the uterine stimulus is able to enlarge as well as to prolong the 

 period of its activity. Clearly, an experiment of this nature is 

 desirable in the guinea pig. 



Attachment of the blastocyst to the uterine epithelium in the 

 guinea pig occurs on the 6th day after fertilization (Blandau, 

 1949) at which time the uterus is sensitive to stimuli producing 

 the decidual reaction. There is, however, no suggestion in 

 Fig. 2 of a change in the growth rate of the corpus luteum at 

 this time and if attachment is responsible for the continuation 

 of luteal growth from the 12th to the 20th days of pregnancy 

 (over which period the corpus luteum is regressing in the 

 non-pregnant animal) it becomes clear that the effects of the 

 stimulus are not apparent for some time later. It would seem 

 probable on the basis of the work of Velardo and colleagues 

 (1953) on the quantitative relationship between the decidual 

 response and extension of luteal function in the rat, and from 

 Nalbandov' s experiments on sheep (loc. cit.), that the stimulus 

 for the continued growth of the corpus luteum of pregnancy 



