FORMATION OF PROTECTIVE EMBRYONIC MEMBRANES 917 



and in this way becomes surrounded completely by the endometrial tissue of 

 the uterus. This condition is known as complete interstitial implantation 

 (fig. 375A-C). 



2. The Placenta and Placentation 



a. Definition 



The process of implantation implies an interaction and attachment between 

 the extra-embryonic membranes and the uterine wall. This area of attachment 

 between maternal and embryonic tissues is called the placenta, and the word 

 placentation denotes the general process effecting this attachment. The word 

 placenta is derived from the Greek and it means a flat cake. It received this 

 name because the human placenta is a flat, rounded mass shaped more or 

 less like a pancake. The placenta may be defined as the association between 

 embryonic and uterine tissues for the purpose of physiological exchange of 

 materials. It is evident that this is a restricted definition applicable only to 

 uterine types of implantation. 



b. Types of Embryonic Tissues Involved in Placentation 



In all vertebrate embryos it is the extraembryonic somatopleure (extra- 

 embryonic ectoderm plus extraembryonic somatopleuric mesoderm) which 

 contacts the uterine mucosa during placentation. In those species which pos- 

 sess a yolk-sac placenta, for example, in the dogfish, Mustelus laevis, the 

 midgut extension of the splanchnopleure which surrounds the yolk unites with 

 the extraembryonic somatopleure to form the embryonic contact (fig. 373A). 

 On the other hand, in the chorio-allantoic placenta of the lizard, Chalcides 

 tridactylus, and in the chorio-allantoic placenta of all eutherian mammals, // 

 is the allantoic evagination of the hindgut which contacts the extraembryonic 

 somatopleure (called the chorion in higher vertebrata) and unites with it to 

 form the embryonic part of the placenta (fig. 373B). However, in all of these 

 instances the epithelium of the extraembryonic somatopleure makes the direct 

 contact with the maternal tissue. Certain exceptions to this general rule appar- 

 ently exist, for m the rabbit during the later stages of gestation, the epitheUum 

 of the chorion may disappear in certain areas, permitting exposure of the fetal 

 blood vessels to the maternal blood (fig. 373E). 



c. Types of Placental Relationships in the Eutherian Mammals 



1) Epitheliochorial Type. If the epithelium of the uterus is not destroyed, 

 and the embryonic tissue merely forms an intimate contact with the uterine 

 epithelium, the placenta is called an epitheliochorial placenta, e.g., pig (fig. 

 373B). Under these conditions the placental area is large and diffuse (see 

 figure 378A). (The placenta of the dogfish, Mustelus laevis (fig. 373 A) is 

 essentially of this type.) 



2) Endotheliochorial Variety. If the epithelium of the uterus is eroded, 



