122 OBSTETRICAL PHYSIOLOGY. 



blood may become impregnated with substances, or impressed with 

 attributes, which will affect the development or modify the constitution 

 of the foetus ; w^hile pernicious matters generated in the latter may prove 

 more or less noxious to the mother. 



Circulation. 



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 

 determining 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 moment when this important organ appears in the form of a 

 cylindrical tube at the commencement of embryonic life, it dilates and 

 contracts alternately, first to receive the venous blood, and then to propel 

 it into the arteries. 



The fluid w^hich is at first received and propelled by the heart is 

 transparent, colourless, 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 dis- 

 position of the aggregations of cells from which these organs are to be 

 developed. It was shown that whilst the outer portions of these 

 aggregations were transformed into the u-alls of the respective cavities, 

 the inner portions appeared 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, colourless, 

 vesicular, spherical cells, full of yellowish particles of a substance like 

 fatty matter. Many of these particles are quadrangular and flattened, 

 and have been called stearine-plates, though their composition is not 

 ascertained ; each cell has a central nucleus, which is not at first very 

 distinct, and the development of these embryo-cells into the complete 

 form of corpuscles is effected by the gradual clearing-up, as if by 

 division and liquefaction, of the contained particles, the acquirement of 

 blood colour 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, whilst 

 they are so numerous 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-coloured, and about 

 l-2500th 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 colourless formative cells 

 is lost ; whilst, on the other hand, there takes place in the vessels of the 

 liver a new production of colourless nucleated cells, which are formed 

 around free nuclei, and which undergo a gradual change, by the pro- 

 duction of colouring matter in their interior, into red nucleated cells. 

 This new formation 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 



