DEVELOPMENT OF THE EMBRYO PROPER. lid 



membrane, but, on the contrary, it undergoes further development in the manner 

 described above. 



The Placenta is the organ by which the connection between the foetus and 

 mother is maintained. It therefore subserves the purposes both of circulation 

 and respiration. It is formed of two parts, as already shown viz. the maternal 

 portion, which is developed out of the decidua vera (serotina), and the foetal 

 portion formed out of the villi of the chorion. Its shape in the human subject is 

 that of a disk, one surface of which adheres to the uterine wall, while the other is 

 covered by the amnion. The villi of the chorion gradually enlarge, forming 

 large projections cotyledons " which each contain the ramifications of vessels 

 communicating with the umbilical (allantoic) arteries and veins of the foetus. 

 These vascular tufts are covered with epithelium, and project into corresponding 

 depressions in the mucous membrane (decidua vera) of the uterine wall. The 

 maternal portion of the placenta consists of a large number of sinuses formed by 

 an enlargement of the vessels of the uterine wall. These bring the uterine blood 

 into close proximity with the villi of the foetal placenta, which dip into the 

 sinuses. The interchange of fluids necessary for the growth of the foetus and for 

 the depuration of the blood takes place through the walls of the villi. but there 

 is no direct continuity between the maternal and foetal vessels. The foetal ves- 

 sels form tufts of capillaries, the blood from which is returned by small veins, 

 which end in tributaries of the umbilical vein. The maternal arteries open into 

 spaces somewhat after the manner of the arteries of the erectile tissues. These 

 spaces communicate with a plexus of veins which anastomose freely with one 

 another, and give rise, at the edge of the placenta, to a venous channel which runs 

 around its whole circumference the placenta! sinus. 



The umbilical cord is formed by the gradual elongation of the belly stalk. It 

 contains the coils of two arteries (umbilical, originally allantoic\ and a single vein, 

 united together by a gelatinous tissue (jelly of Wharton). There are originally 

 two umbilical veins, but one of these vessels becomes obliterated, as do also the 

 two omphalo-mesenteric arteries and veins and the duct of the umbilical vesicle, 

 all of which are originally contained in the belly stalk. The permanent struc- 

 tures of the cord are. therefore, furnished by the allantois. 



In this manner the human embryo eventually becomes surrounded by three 

 membranes : (1) the amnion, derived from the outer layer of the mesoblast and the 

 epiblast: ('2) the ehorion, formed from the false amnion (which is derived from 

 the outer layer of the mesoblast and the epiblast), and (3) the decidua, derived 

 from the mucous membrane of the uterus. 



Development of the Embryo proper. The further development of the embryo 

 will, perhaps, be better understood if we follow as briefly as possible the principal 

 facts relating to the chief parts of which the body consists viz. the spine, the 

 cranium, the pharyngeal cavity, mouth, etc., the nervous centres, the organs of 

 the senses, the circulatory system, the alimentary canal and its appendages, the 

 organs of respiration, and the genito-urinary organs. 1 The reader is also 

 referred to the chronological table of the development of the foetus at the end of 

 this section. 



Development of the Spine. We have already traced the first steps in the 

 formation of the spine : (1) The looping up of two longitudinal folds from the 

 cells of the epiblast on either side of the primitive streak, so as to form a groove, 

 and the gradual growing together of these ridges (laminae dorsales) so as to con- 

 vert the groove into a canal, which is lined by epiblast, and out of which the 

 spinal cord is developed. (2) The formation in front of this groove of a con- 

 tinuous cellular cord enclosed in a structureless sheath, the notochord or chorda 

 dorsalis (Fig. 89). The notochord extends from the cephalic to the caudal 



1 The scope of this work only permits the briefest possible reference to these subjects. Those 

 who wish to study the subject of embryology in more detail are referred to Kolliker's Entwickdunqg- 

 y?*-~hichte ; to vol i. pt. 1, of the tenth edition of Quain's Anatomy; or to the works of Professors 

 Minot and Hertwig. 



