THE MEMBRANES AND APPENDAGES OF THE FCETUS. 53 



cleft three eminences appear. From the eminences on the two arches, and the 

 skin immediately posterior to the eminences on the hyoid arch, are formed the 

 various parts of the auricle, but the exact part played by the individual 

 eminences in the human subject is as yet a matter of some doubt. 



THE PROTECTION AND NUTRITION OF THE EMBRYO DURING 

 ITS INTRA-UTERINE EXISTENCE. 



Whilst it is passing down the uterine tube, and for a brief period after it 

 enters the uterus, the zygote, or impregnated ovum, depends for its nutrition 

 upon the yolk granules (deutoplasm) embedded in its cytoplasm, and upon the 

 fluid medium surrounding it which is secreted by the walls of the uterine tube 

 and the uterus. 



As the human ovum is very small, and as it contains but little deutoplasm, its 

 nutrition is practically dependent, almost from the first, upon external sources 

 of supply. The urgent necessity for the formation of adequate arrangements 

 whereby the external sources may be utilised leads to the early establishment 

 of an intimate connexion between the zygote and the mother, which is one of the 

 characteristic features of the development of the human embryo. 



During the third week after fertilisation, as the embryo is beginning to be 

 moulded from the embryonic region, and before the paraxial mesoderm commences 

 to separate into mesodermal somites, a primitive heart and the rudiments of 

 some well-defined blood-vessels are distinguishable in the embryo; but the 

 details of the development of the vascular system and the establishment of the 

 embryonic circulation cannot be well understood until the formation and structure 

 of a group of closely associated extra-embryonic organs or appendages, derived 

 from the zygote, has been considered. 



This group includes the chorion, the placenta, the amnion, the umbilical cord, 

 and the yolk-sac. 



THE MEMBRANES AND APPENDAGES. 



The Chorion. It has already been noted that when the zygote becomes a 

 blastula it consists of three vesicles, a large vesicle enclosing two smaller vesicles 

 and a mass of primary mesoderm (Fig. 29). 



The wall of the large vesicle is composed of trophoblast (trophbblastic ectoderm), 

 and its inner surface is in direct contact with the primary mesoderm. 



A little later a cavity, the extra-embryonic ccelom, appears in the primary 

 mesoderm, separating it into two layers, .one lining the inner surface of the tropho- 

 blast and the other covering the outer surfaces of the two inner vesicles (Figs. 

 70, 71). 



As soon as the extra-embryonic coelom is established the chorion is formed ; 

 it consists of the trophoblast and its inner covering of mesoderm. 



In the meantime the trophoblast has differentiated into two layers, an inner 

 cellular layer, and an outer plasmodial layer. In the plasmodial layer cell 

 territories are not denned, and it consists, therefore, of nucleated protoplasm. 



The differentiation of the trophoblast into two layers occurs after the zygote 

 is embedded in the mucous membrane of the uterus which is modified for its 

 reception and which, after the modification has occurred, is called the decidua. 



As development proceeds the trophoblast increases in thickness and it invades the 

 decidua. As this invasion occurs the plasmodial layer of the trophoblast becomes 

 permeated with spaces which are continuous with the lumina of the maternal 

 blood-vessels in the decidua, and are filled with maternal blood. 



By means of the spaces the plasmodial trophoblast is separated into branching 

 processes which intervene between ' the blood-filled spaces. The processes are 

 the primary chorionic villi, and they soon develop -cellular interiors (Fig. 72). 



After a time the primary villi are invaded by the chorionic mesoderm, and are 

 thus converted into the secondary chorionic villi, which become vascularised by the 



