62 INTRODUCTION TO SEXUAL PHYSIOLOGY 



wholly concerned with the nutrition and development of the 

 embryo. This structure is the extra-embryonic ectoderm or 

 trophoblast ; it forms the outer layer of the chorion, an organ 

 of great importance, since it is by means of the chorion and the 

 villi, which subsequently grow out from it, that the embryo 

 is connected with the maternal placenta which is formed from 

 the uterine mucous membrane. The trophoblast becomes 

 differentiated into two layers, an outer one consisting of a layer 

 of protoplasm studded with nuclei but not divided into cells — 

 the syncytiotrophoblast (giving rise later to the syncytium), and an 

 inner one with cell outlines — the cytotrophoblast. The mode of 

 formation of the inner portion of the chorion is described below. 



The lower or innermost layer of cells constituting the formative 

 cell mass above referred to, at an early stage becomes flattened 

 out, giving rise to the endoderm, and from the endoderm the yolk 

 sac is formed as a little closed vesicle within the blastocyst. Part 

 of its cavity (the archenteron) is eventually included in the embryo, 

 becoming the alimentary canal. In man and the other Primates 

 the yolk sac does not appear to exercise any function as an organ 

 of embryonic attachment, such as it does in most mammalian 

 orders, and notably in marsupials, in many species of which it 

 forms the sole foetal placental organ (or organ of foetal nutrition). 



The third germinal layer or mesoderm arises from the sides 

 of the embryonic area of the blastocyst, between the embryonic 

 ectoderm and the endoderm (see below). As it spreads out it 

 splits into two layers with a space between them. The external 

 layer is called the somatopleur and the internal layer the splanch- 

 nopleur. The former adheres to the inner aspect of the tropho- 

 blast, which is composed of extra -embryonic ectoderm and forms 

 with it the chorion or diplotrophoblast. The splanchnopleur is 

 applied externally to the endodermic wall of the yolk sac. The 

 space between the two layers is the extra-embryonic coelom, 

 which intervenes between the chorion and the yolk sac. 



In the rabbit and many other mammals the amnion or inner 

 of the foetal membranes arises by the formation of folds of extra- 

 embryonic ectoderm together with the somatopleur in con- 

 junction with it, the splanchnopleur taking no part in the process. 

 The extra-embryonic coelom is continued in the folds, each 

 of which contains two layers. The folds grow up over the dorsal 

 surface of the embryo and eventually meet and fuse, and when 



