FORMATION OF BLOOD-CORPUSCLES. li 



In the bird. In the egg of the bird, the first appearance of blood-corpuscles, as 

 well as of blood-vessels, is seen in the blastoderma, or germinal membrane, a structure 

 formed by the extension of the cicatricula in the early stages of incubation. The 

 commencing embryo, with its simple tubular heart, is seen in the middle of this 

 circular membrane, and blood-vessels, containing blood-corpuscles, appear over a 

 great part of its area. These first vessels, therefore, though connected with the 

 heart, and intended to convey nutriment to the embryo, are formed in an exterior 

 structure ; but, in a somewhat later stage, blood-vessels and corpuscles are developed 

 in various textures and organs within the body. The formation of blood-corpuscles 

 in the vascular area of the blastoderma has been sedulously investigated by various 

 inquirers ; and from their concurrent statements, we learn that these corpuscles, at a 

 certain stage of their progress, are rounded bodies, larger than the blood-disks of the 

 adult. They contain a granular nucleus, and are quite devoid of colour. These 

 spheroidal colourless corpuscles in their further advancement become flattened, and 

 assume an oval figure. While undergoing these changes of form, they acquire a 

 red colour, which is at first faint and yellowish, but gradually deepens. 



As to the earlier part of the process the production of the above-mentioned round 

 cells, whose subsequent conversion into coloured oval disks has just been described 

 it has been held that the cells which form the substance of the blastoderma and 

 embryo partly pass directly into blood-corpuscles, and partly generate the latter by 

 fissiparous multiplication. 



In man and mammalia. In the embryo of man and mammalia the primitive 

 blood-corpuscles are round, nucleated, colourless bodies, as in the cases above de- 

 scribed. Their substance, originally granular, speedily clears up and acquires colour, 

 and thus they appear as nucleated red corpuscles, of spheroidal shape, and of much 

 larger size than the future red disks. They are embryonic cells, most probably 

 loosened from each other and set free in the excavation of the originally solid vessels 

 in the blastoderma and embryo-body ; and, both in their primitive state and after 

 acquiring colour, they increase in number by fissiparous multiplication, as represented 

 in fig. XL, p. xvii. These large nucleated red and colourless corpuscles, continuing 

 to increase in number, constitute the earliest and, for a time, the only corpuscles in 

 the embryo-vessels. But their multiplication is soon arrested, and a new epoch in 

 blood-formation begins with the development of the liver. The blood which returns 

 to the embryo charged with fresh material of nutrition from the maternal system, 

 has then to pass, at first entirely, afterwards in great part, through the vessels of the 

 liver ; and it would seem that henceforth colourless nucleated corpuscles are produced 

 in that organ and poured abundantly into the general mass of blood by the hepatic 

 veins. It is probable that the liver continues its haemapoietic or blood-forming 

 function throughout foetal life ; but, in the meanwhile, the spleen and lymphatic 

 system have also begun to produce pale corpuscles, and in after periods supersede the 

 liver in that ofiice. These corpuscles, either immediately or after fissiparous multi- 

 plication, acquire colour like the first those from the liver and spleen probably in 

 great part before they leave these organs and are converted into nucleated red 

 corpuscles. The nucleated red corpuscles thus produced are gradually converted 

 into, or at least succeeded by, smaller disk-shaped red corpuscles without nuclei, 

 having all the characters of the blood-disks of the adult. This transition or sub- 

 stitution begins early, and proceeds gradually, until at length, long before the end of 

 intrauterine life, the nucleated red corpuscles have altogether vanished. 



Throughout life the mass of blood is subject to continual change ; a portion of it is 

 constantly expended, and its place taken by a fresh supply. It is certain that the 

 corpuscles are not exempted from this general change, but it is not known in what 

 manner they are consumed, nor has the process been fully traced by which new ones 

 are continually formed to supply the place of the old. With regard to the latter 

 question, it may be stated, that the explanation which has hitherto found most favour 

 with physiologists is, that the corpuscles of the chyle and lymph, passing into the 

 sanguiferous system, become the pale corpuscles of the blood ; and that these last are 

 converted into red disks. Pale corpuscles are also generated in the spleen, and, after 

 part of them have changed into red disks, pass directly into the blood, independently 

 of those derived from the chyle and lymph. As to the manner in which the pale 

 corpuscles are transformed into the red, there is considerable difference of opinion. 



