858 



SPERM, OVA, AND PREGNANCY 



Fig. 14.21. Living guinea pig blastocyst removed appioximatel}' one-half hour before at- 

 tachment to the endometrium. The blastocyst is slightly rotated to show the extensive pro- 

 toplasmic projections at the abembryonal pole. X 900. 



scribed in fixed preparations of a number 

 of genera of grovnid squirrels and chip- 

 munks (Lee, 1903; Mossman, 1937). Al- 

 though the extension of conclusions based 

 on the study of one species to other species 

 is precarious, it is possible that the same re- 

 lationship of the attachment cone to the 

 zona pellucida exists in other forms (Moss- 

 man, 1937). Boving noted a change in the 

 viscosity and adhesiveness of the rabbit egg 

 investments at the time of implantation 

 which he attributed to local alkalinity (pH 

 9) released from one or more regions of 

 the abembryonic hemisphere. Following ad- 

 hesion, the outer investments of the blasto- 

 cyst in this area disintegrate. Inasmuch as 

 remnants of the membranes are sometimes 

 observed in the areas between the implant- 

 ing blastocysts, their final removal ap- 



jiarently is similar to that described for the 

 guinea pig. When the membranes have been 

 shed the abembryonic trophoblast adheres 

 to the uterine epithelium, particularly in 

 areas where blood vessels are subjacent 

 to the epithelium. The trophoblast pene- 

 trates the epithelium by displacement, and 

 the invasion of the stroma at first is not 

 destructive. 



O. BLASTOCYST EXP.\XSION 



In the guinea pig, rat, mouse, and ham- 

 ster, the diameter of the blastocyst at the 

 time of attachment is approximately the 

 same as that of the tubal ova. In these 

 species implantations are more or less regu- 

 larly spaced but not invariably so, because 

 placental fusion occurs frequently. Thus 

 there does not seem to be the same purpose- 



