508 URO-GEN1TAL SYSTEM. 



work and expends no force; it has not even to produce 

 heat, it receives that from the mother. It takes alimentary 

 materials only for the building of tissues and the devel- 

 opment of organs ; consequently the difference in the char- 

 acter of its venous and arterial blood is not very great, 

 and by no means the same as in the arterial and venous 

 blood of adult life. However, oxidation, no matter how 

 feeble it may be, is produced in the embryo ; thus, the heart 

 performs its work and must occasion products of combustion ; 

 moreover, every formation of tissue is accompanied by phe- 

 nomena of combustion, which should give rise to excremen- 

 titial products. These products are eliminated principally by 

 the liver and urinary organs (first the Wolffian body and 

 then the kidneys) ; the liver is also very much developed in 

 the embryo, and up to a certain point it may replace the 

 lung as an organ for the excretion of organic waste. A 

 certain amount of urea is also contained in the bladder of 

 the embryo, which is thrown off into the amniotic cavity. 

 Consequently, the amniotic fluid contains at the close of the 

 embryonic life a large number of excrementitial products, 

 because in addition to the urine there are products resulting 

 from the desquamation of the skin. 



II. Development of the body of the embryo. 



If we bear in mind what has preceded in regard to the 

 formation of the umbilical vesicle (pp. 500 and 503) we shall 

 also understand how this vesicle, in consequence of a peculiar 

 strangulation, is separated from the common blastodermic 

 vesicle (p. 587) ; the borders of the germinal space or area, as 

 well as its cephalic and caudal extremities or hoods, drawn 

 along by this strangulation or constriction, form by their 

 curvatures the sides as well as the cephalic and caudal hoods 

 (Fig. 137, 139, 140), which unite and form a cavity. This 

 cavity might be likened to the hollow or interior of a slipper, 

 and communicates with that of the umbilical vesicle, as we 

 have before stated (Fig. 139, p. 501). This is the primary 

 cavity of the embryo, or rather its intestinal cavity (Fig. 

 137, 12). To complete this rough sketch of embryology we 

 will proceed to the consideration of the two grand systems, 

 the nervous system and that of the circulation. 



a. Central Nervous 'System. As soon as the germinal 

 space or area has assumed the form of an elongated spot 

 (like the sole of a slipper) a central longitudinal line, called 



