C. R. AUSTIN AND M. W. H. BISHOP 79 



ductive tract, notably the cervical mucus ( Pommerenke and 

 Viergiver, 1946; Glover and Scott Blair, 1953) it would be unex- 

 pected to find that spermatozoon transport occurred with equal 

 rapidity at all phases of the estrous cycle. However, Green and 

 Winters (1935), Schott and Phillips (1941), and Starke (1949) 

 all consider that the speed of transport in the sheep is independ- 

 ent of ovarian activity. Rowson (1955), on the other hand, ex- 

 presses doubt that this is so in the cow, and Warbritton, McKen- 

 zie, Berliner, and Andrews (1937) suggest that conditions for 

 spermatozoon transport in the ewe are optimal 10 to 12 hours 

 after the beginning of estrus. In the rat the passage of spermato- 

 zoa occurs more readily late in estrus (Braden and Austin, 

 1954a ) . During pseudopregnancy in the rabbit, conditions do not 

 favor the fertilization of eggs (Wislocki and Snyder, 1931, 1933; 

 Mm-phree, Warwick, Casida, and McShan, 1947; Boyarsky, Bay- 

 lies, Casida, and Meyer, 1947) and fewer spermatozoa reach the 

 uterus and tubes than during estrus (Austin, 1949). Unfavorable 

 conditions obtaining in the tract at other times than estrus may 

 constitute a defense against the possibility of fertilization in preg- 

 nancy, an occurrence that could lead to superfetation. That su- 

 perfetation can arise has been shown experimentally in the rat 

 (Canivenc, Doruville, and Mayer, 1953). 



From what has been said it appears that the mechanical and 

 physical attributes of the environment of fertilization tend to exert 

 opposing effects upon the spermatozoa. The specialized move- 

 ments of the tract, accompanied in certain instances by pressure 

 gradients, have the effect of quickly and efficiently transporting 

 spermatozoa to the site of fertilization. On the other hand, attri- 

 butes such as the narrow lumen of certain regions, the accumu- 

 lations of fluid, and the large mucosal surface areas exert a dilut- 

 ing influence and greatly diminish the number and density of 

 spermatozoa approaching the site of fertilization. 



Maintenance of Spermatozoa in the Female Tract 



Immediately following ejaculation, the spermatozoa, aheady 

 suspended in the male accessory secretions, are further diluted 

 by admixture with the secretions of the female tract. The propor- 



