854 



SPERM, OVA, AND PREGNANCY 



with increasing amplitude. Thereafter, con- 

 tractions occur at intervals of 1 to 2 minutes 

 followed by prompt relaxation. In contrast, 

 similar relaxation was not observed in uter- 

 ine strips under the influence of progester- 

 one. Instead they slowly shorten. 



Ivy, Hartman and Koff (1931j observed 

 that muscular contraction waves in the 

 monkey uterus originate from an area 

 slightly ventral and cranial to the insertions 

 of each of the oviducts and then proceed 

 medially to meet in the midline. They con- 

 cluded that in the monkey the area of the 

 endometrium where implantation usually 

 occurs is affected by contractions to a lesser 

 extent than the remainder of the uterus. 



Nicholas (1936) interposed a section of 

 duodenum into the rat's uterus and found 

 embryos in the lower uterine segment. Lim 

 and Chao (1927) reversed the middle por- 

 tion of one or both cornua of the rabbit and 

 reported that pregnancy was not prevented. 



Markee (1944) introduced sea urchin 

 eggs, celloidin balls, and glass beads into the 

 tubal ends of rabbit cornua and observed 

 their distribution at varying intervals from 

 estrus to 10 days after ovulation. He found 

 that the sea urchin eggs were distributed 

 most evenly in the uteri of cstrous rabbits, 

 especially at the time of ovulation. Fairly 

 good distribution was recorded at 5 days 

 and poor distribution at 10 days after ovu- 

 lation. As noted below, none of these inert 

 objects or sea urchin eggs expand with time 

 as do rabbit blastocysts before attachment. 

 It is doubtful that the movements of these 

 objects in utero could be considered as the 

 normal state of affairs in the transport of 

 blastocysts. In order to study this problem 

 further, Markee observed uterine contrac- 

 tions directly through a glass window which 

 had been sewn into the abdominal wall. 

 Three types of contractions were observed 

 during estrus and for 5 days after ovulation : 

 (1) local ring-type contractions persisting 

 for approximately 10 seconds, (2) peristal- 

 tic contractions proceeding throughout the 

 length of the cornu, and (3) antiperistaltic 

 waves of approximately the same intensity 

 as the peristaltic contractions. After the 5th 

 day, the peristaltic and antiperistaltic con- 

 tractions decreased greatly in amplitude 

 and in the length of their excursions. 



Recent studies on the mechanisms con- 

 tributing to the distribution of the implant- 

 ing rabbit blastocysts have directed at- 

 tention to the possibility that both physical 

 and chemical interactions between the blas- 

 tocyst and uterus are important (Boving, 

 1952a, b, 1954, 1956, 1959). Boving has 

 found that by 7 days post coitum, rabbit 

 blastocysts have achieved an almost even 

 distribution, not only with reference to 

 the space between them, but also with 

 respect to the entire length of the uter- 

 ine cornu (Fig. 14.18). If the num- 

 ber of blastocysts in utero varies, the 

 spacing is nevertheless appropriate to 

 their number. The cornua reacts to 

 the presence of each blastocyst and po- 

 sitions it in relation to all other blasto- 

 cysts present until a remarkably even dis- 

 tribution is achieved by the 7th day post 

 coitum. There is evidence from the work 

 on the rabbit at least that the movement 

 and i)ositioning of blastocysts in utero 

 coincide with their increase in size. Rabbit 

 blastocysts of approximately 1-mm. size are 

 propelled much more slowly than blasto- 

 cysts or glass beads 3 to 6 mm. in diameter. 

 Boving suggested that each blastocyst acts 

 as a localized stimulus which initiates the 

 propulsive muscular activity and that the 

 size of the blastocysts determines the way 

 in which the myometrium responds. Cessa- 

 tion of positioning is coincident with a local 

 loss of uterine tone and a ballooning out 

 of the antimesometrial wall to form a 

 "dome." 



The blastocysts of the leporid family of 

 rodents, the carnivores, some insectivores, 

 and bats undergo considerable expansion 

 in the uterine cavity before and at the time 

 of attachment. In these animals, then, the 

 spacing of the blastocysts may be arranged 

 according to Boving's theory that myogenic 

 uterine contraction is the effector of both 

 propulsion and spacing. 



As mentioned earlier, during the 6th and 

 7th days after copulation in rabbits, the 

 expanded blastocysts occupy a distended, 

 antimesometrial "dome" caused by a local 

 decrease in uterine muscle tone. From in 

 vivo observations of the pregnant uterus, 

 Boving (1952b, c) observed that the blas- 

 tocysts of the rabbit undergo a rotational 



