114 OBSTETRICAL PHYSIOLOGY. 



Ciradation. 



In order that the nutritive matters absorbed by the placental villi may 

 reach the foetus and be circulated in its body, it is necessary that a deter- 

 mining and regulating power should be brought into play at an early period 

 in the existence of the foetus. This power appears with the formation of 

 the contractile organ which has been named the heart. From the mo- 

 ment when this important organ appears in the form of a cylindrical tube 

 at the commencement of embryonic life, it dilates and contracts alter- 

 nately, first to receive the venous blood, and then to propel it into the 

 arteries. 



The fluid which is at first received and propelled by the heart is trans- 

 parent, colorless, and destitute of morphological elements, and the organ 

 itself exists in its most primitive form. As has been stated, the situation 

 of the heart and the course of the principal trunks of the vascular area, 

 are early visible, and are marked by the peculiar disposition of the aggre- 

 gations of cells from which these organs are to be developed. It was 

 shown that whilst the outer portions of these aggregations were trans- 

 formed into the walls of the respective cavities, the inner portions ap- 

 peared to deliquesce, and partly to remain as isolated cells floating in the 

 resulting fluid. These isolated cells are supposed to be the first blood 

 corpuscles. They are large, colorless, vesicular, spherical cells, full of 

 yellowish particles of a substance like fatty matter. Many of these parti- 

 cles are quadrangular and flattened, and have been called stearine-plates, 

 though their composition is not ascertained ; each cell has a central nu- 

 cleus, which is not at first very distinct, and the development of these 

 embryo-cells into the complete form of corpuscles is effected by the grad- 

 ual clearing-up, as if by division and liquefaction, of the contained particles, 

 the acquirement of blood color and of the elliptical form, the flattening 

 of the cell, and the more prominent appearance of the nucleus. 



In tracing the development of the red-corpuscles of the blood, it is found 

 that at first their nuclei have no envelope, but contain nucleoli ; that they 

 present all the characters of pale elementary cells, w^hilst they are so nu- 

 merous as to give the blood a whitish hue. When more fully developed 

 they acquire a cell and a reddish tint, and at a later stage are circular, 

 thick, and disc-shaped, full-colored, and about 1-2 500th part of an inch 

 in diameter ; their nuclei are central, circular, very little prominent on 

 the surface of the cell, and apparently slightly granular or tuberculated. 



When the liver begins to be formed, the multiplication of blood-cells in 

 the entire mass of the blood ceases, and in a short time all trace of the 

 development of the red from the original colorless formative cells is lost, 

 whilst, on the other hand, there takes place in the vessels of the liver a 

 new production of colorless nucleated cells, which are formed around 

 free nuclei, and which undergo a gradual change, by the production of 

 coloring matter in their interior, into red nucleated cells. This new for- 

 mation of blood corpuscles in the liver continues to take place during the 

 whole period of foetal life ; but whether these nucleated cells themselves 

 undergo transformation into the non-nucleated discs which constitute a 

 gradually increasing proportion of the corpuscular components of the 

 blood during the latter period of embryonic life, or whether these are 

 formed only by the metamorphosis of lymph-corpuscles, has not yet been- 

 determined. 



From the manner in which the circulation is carried on during foetal 



