64 HUMAN EMBEYOLOGY. 



To meet this necessity the blood vascular system is formed. The system is 

 essentially an irrigation system. In its earliest stages it consists of a series of 

 vessels, the blood-vessels, all of which contain a corpuscle-laden fluid, called blood. 

 The blood is kept circulating, in the early stages, by the rhythmical contraction 

 of the walls of the vessels, but, after a short time, parts of the vessels are 

 developed into a muscular organ called the heart. After the heart is established 

 the continuance of the circulation of the blood depends upon the regular con- 

 tractions of the muscular substance of its walls. 



The corpuscular portions of the blood and the walls of the blood-vessels are 

 formed from the cells of the zygote, but it is obvious, in the early stages at all 

 events, that the fluid portion of the blood must be obtained from the mother. It 

 is necessary, therefore, both for this purpose and for the facilitation of interchanges 

 between the foetal and maternal blood streams, that the foetal blood-vessels should 

 be brought into close association with the maternal blood at an early period. It is 

 for this purpose, among others, that large spaces appear in the trophoblast ; that 

 the spaces become filled with blood from maternal vessels which have been opened 

 up by the destructive action of the trophoblast cells ; and that the spaces are 

 afterwards invaded by the chorionis villi, which carry in their interiors branches 

 of the blood-vessels of the embryo. As soon as the intimate relationship between 

 the chorionic villi and the maternal blood is established fluids can readily pass 

 from the^ maternal to the foetal vessels, and there can be no doubt that both food 

 and oxygen pass from the maternal to the foetal blood through and by the agency 

 of the trophoblastic epithelium, whilst, at the same time, waste products of foetal 

 metabolism pass from the foetal to the maternal blood. 



The germs of the vascular system are a series of cells arranged in strands 

 which constitute, collectively, the angioblast. They appear between the entodermal 

 and the mesoderrnal layers of the wall of the yolk-sac, and, therefore, entirely outside 

 the embryo ; but it is not certain whether they are derived from the mesoderm or 

 from the entoderm. 



Origin of Blood Corpuscles. After a time the angioblast separates into two 

 parts, (1) the peripheral cells of the strands which form the endothelial walls of the 

 primitive blood-vessels, and (2) the central cells which become the primitive blood 

 corpuscles or mesamoeboids (Minot). 



The mesamoeboids are colourless cells with large nuclei and a relatively small 

 amount of protoplasm; from them are formed, either by transformation or 

 division, (1) the erythrocytes, which are coloured blood corpuscles, and (2) nucleated 

 colourless corpuscles. The erythrocytes are nucleated cells with a homogeneous 

 protoplasm which contains the substance, called haemoglobin, upon which the 

 yellowish-red colour of the cells depends, and from them are derived the fully 

 developed red corpuscles. 



The primitive erythrocytes, the ichthyoid cells of Minot, are transitory structures 

 in mammals, but they are the permanent red blood cells of the ichthyopsida (fishes 

 and amphibia). They are succeeded by the sauroid blood cells (Minot), which 

 represent the permanent corpuscles of reptiles and birds, and which are distinguish- 

 able from the ichthyoid cells by their smaller size and more deeply-staining nuclei. 



The sauroid blood cells are replaced by the blood plastids, which are young non- 

 nucleated red corpuscles. According to some observers the blood plastids are 

 sauroid cells which have lost their nuclei, whilst other investigators believe the 

 blood plastids to be the nuclei of sauroid cells. Whatever their origin, they become 

 converted into permanent red blood corpuscles by transformation from the spherical 

 to a cup-shaped and later to a biconcave form. 



The young red blood cells are therefore the ichthyoid cells, those progressively 

 older are sauroid cells, blood plastids, and blood corpuscles. 



The colourless, nucleated corpuscles white blood corpuscles are much less 

 numerous than the coloured corpuscles in the adult blood. They appear to be 

 derived from the mesamoeboids, though it is possible that they are also formed by 

 ordinary mesoderm cells, and as regards those formed from mesamoeboids it is not 

 certain whether a rnesamceboid cell can by division produce both erythrocytes and 

 white corpuscles, or whether it must produce one or the other. (See note 5, p. 79.) 



