904 CARE AND NOURISHMENT OF THE DEVELOPING YOUNG 



of the embryo. All vertebrate embryos, from the fishes to the mammals, are 

 protected normally by the primary embryonic membrane during the period 

 of cleavage, and, in many fishes and amphibians this membrane functions 

 until the time when the embryo hatches and assumes a free-living existence. 



b. Tertiary Egg Membranes 



1) Mammals. The lengths of the Fallopian tubes of different mammalian 

 species vary considerably. In the mouse, rabbit, human, and sow, the Fal- 

 lopian tubes vary in length, not only from species to species but also from 

 individual to individual within the species. Yet, the time of passage of the 

 egg through this region of the reproductive duct approximately, for all four 

 species, is from 3 to SVi days. On the other hand, the length of the uterine 

 tube of the opossum may be from 5 to 10 times that of the mouse, yet the 

 time consumed in egg transport in the former species is about 19 to 24 hours. 

 Moreover, in the sow and mouse, evidence has been accumulated which tends 

 to show that egg transport through the middle portion of the uterine tube 

 is slower than that of the portion near the infundibulum or of the part near 

 the uterus (Anderson, '27; Lewis and Wright, '35). In the Monotremata, 

 Flynn and Hill ('39, p. 540) conclude that "passage through the tube must 

 be fairly rapid." In all these instances, the rate of egg travel through the 

 uterine tube appears to be dependent upon necessary developmental changes 

 within the cleaving egg and functional changes within the uterus and the 

 uterine tube. In other words, the rate of egg propulsion through the Fallopian 

 tube varies with the species. The time consumed in transit is not related to 

 the length of the tube, but is correlated with changes in the uterus, pre- 

 paratory to receiving the egg at a proper developmental stage. 



The deposition of protective enveloping coats around the egg during egg 

 passage through the Fallopian tube is encountered in certain mammals. In 

 the monotremes, a rather dense, albuminous coat is deposited around the egg 

 in the upper two thirds of the Fallopian tube, and a clearer, more fluid secre- 

 tion is deposited around the egg by the glandular cells in the posterior third 

 of the tube (Flynn and Hill, '39). A leathery shell is formed around the egg 

 and these albuminous coats in the posterior segment or uterus. In the opossum, 

 a dense albuminous coating forms around the egg during its passage down 

 the upper part of the Fallopian tube, while a thin much tougher membrane 

 is added around the outside of the albuminous material in the tube's lower 

 part. In the rabbit, a thick albuminous coating is deposited around the egg 

 as it passes downward within the Fallopian (uterine) tube. Therefore, forma- 

 tion of protective egg envelopes may be regarded as a specific function of the 

 Fallopian tube during egg passage in some mammals. 



The reactions of the developing egg within the uterine portion (uterus) 

 of the reproductive duct in the higher mammals are dramatic events in which 

 the embryo develops special contacts with the uterine wall. In some cases. 



