918 CARE AND NOURISHMENT OF THE DEVELOPING YOUNG 



and the embryonic tissue (i.e., chorionic epithehum) comes in contact with 

 the endothelium of the maternal blood vessels, the attachment is called an 

 endotheliochorial placenta (e.g., dog, cat, and other Carnivora, figure 373C). 

 As the placental attachment becomes more intimate the placental area be- 

 comes restricted. Compare figure 378A and B with C, D and E. 



3) Endotheliochorial Plus Syndesmochorial Placenta. In the Ungulata 

 (cows, sheep, goats) the placenta is an extensive affair similar to that of the 

 pig. However, the attachment between embryonic and maternal tissues occurs 

 in certain areas known as cotyledons (fig. 378B). In parts of these cotyledons 

 the association of maternal and embryonic tissue is of the endotheliochorial 

 variety, but in other areas of the cotyledons only the epithelium of the uterus 

 disappears, leaving the chorionic epithelium of the extra-embryonic tissue in 

 contact with the connective tissue of the uterine wall. A condition where the 

 chorionic epithelium makes contact with the connective tissues of the uterine 

 wall is called a syndesmochorial relationship. 



4) Hemochorial Placenta. In the rodents, primates (including man), shrews, 

 moles, and bats the endothelium of the maternal blood vessels is destroyed 

 by the erosive activity of the embryonic tissues, and the chorionic epithelium 

 of the embryonic portion of the placenta comes directly in contact with the 

 maternal blood (fig. 373D). This type of association is known as a hemo- 

 chorial placenta. 



5) Hemoendothelial Placenta. In the rabbit, the initial contact of the fetal 

 tissues with the uterine epithelium forms an epitheliochorial relationship. 

 Still later it becomes, after erosion of maternal tissue, a hemochorial condition, 

 and finally, during the latter phases of pregnancy, even the chorionic epithelium 

 disappears, leaving the endothelium of the embryonic blood vessels in contact 

 with the maternal blood (fig. 373E). This type of association is the most inti- 

 mate placental contact known and it is called a hemoendothelial relationship. 



3. Implantation of the Human Embryo 

 a. Preparation for Implantation 



In all cases of uterine care of the developing egg, the uterus must be pre- 

 pared for the event. This preparation is induced by the activities of the ovarian 

 hormones (see Chapter 2 and figures 53 and 59). Implantation of the embryo 

 occurs in the early luteal phase of the reproductive cycle when the endometrial 

 mucosa is in an optimum condition for the reception of the developing egg. 



b. Implantation 



As indicated above, p. 904, the process of egg transport down the Fallopian 

 tube occurs at a rate which permits the developing egg (embryo) and the 

 uterine tissue to prepare themselves for the implantation event. About three 

 to three and one-half days elapse during the passage of the egg through the 



