856 



SPERM, OVA, AND PREGNANCY 



er's fluid at first caused a swelling of the 

 zonae, and the ordinarily smooth outer 

 contour became wavy and fringe-like. In 

 measuring the hydrogen ion concentration 

 of the fluids in the vicinity of deciduomata 

 of the rat, values as low as pH 5.7 were 

 recorded. Such values w^ere of sufficient 

 acidity to effect the gradual softening of 

 the zona pellucida. Pincus and Enzmann 

 (1936) also measured the pH of uterine 

 luminal fluids in pseudopregnant rats and at 

 no time observed values below 6.7. From 

 Hall's work it was concluded that "as the 

 decidua develops around the implanting egg 

 and as the metabolic activities of the divid- 

 ing blastocyst increase, the fluid bathing 

 the blastocyst may become sufficiently acid 

 to be a factor in the removal of the oo- 

 lemma." Fertilized mouse ova, transplanted 

 to the anterior chamber of the eye, lost 

 their zonae independently of a change in 

 hydrogen ion concentration of the envi- 

 ronmental fluids <Fawcett. Wislocki and 



Waldo, 1917). Other factors which alter 

 the physical properties of the secondary 

 and tertiary membranes were described 

 earlier. At this point, how^ever, it is im- 

 portant to direct attention to Lutwak- 

 Mann's (1959) recent comments on the 

 toughness and resilience of the zona in the 

 rabbit and the difficulty in dissolving it ex- 

 cept by harsh enzyme and chemical means. 

 Results obtained during work on the 

 guinea pig and rabbit have prompted in- 

 vestigators to think of other methods by 

 which the zona pellucida and other invest- 

 ing membranes might be shed. Remnants 

 of the zona pellucida have been found ad- 

 hering to the blastocyst wall in sections of 

 early implanting guinea pig ova (von Spee, 

 1901; Maclaren and Bryce, 1933). This 

 fact indicates that the zona pellucida is 

 not uniformly lost by chemical action of 

 the fluids of the uterus. In the guinea pig 

 the abembryonal pole of the blastocyst 

 first makes contact with the endometrial 



Fig. 14.19. Photomicrograph of a living guinea pig blastocyst removed on the 6th day after 

 ovulation. The inner cell mass is directed towards the top of the page. Abembryonal cells 

 form but a single layer (compare with Figure 14.17, 2). X 900. 



