THE BLOOD. 185 



a diminution of the fibrin, which is supposed to depend upon the 

 plates for its source. Blood-plates are increased in leukaemia. 



Hcemaconien are smaller than the blood-plates, and appear to 

 be composed of portions of the protoplasm of leucocytes. They 

 contain proteid and fatty matters. 



FORMATION OF RED BLOOD=CORPUSCLES. 



The red corpuscles, like every other portion of the economy, 

 perform their allotted task and round of existence, to finally die and 

 disappear. 



The origin of the red corpuscle as to time may be spoken of as 

 that which occurs during intra-uterine life and that occurring during 

 extra-uterine life. 



During Intra-uterine Life. The corpuscles which first appear 

 in the human embryo owe their existence to a very simple origin. 

 They differ in some respects from those that appear later during 

 intra-uterine life, and very materially from those formed during life 

 outside of the uterus. 



The wall of the yelk-sac, situated entirely outside of the body 

 of the embryo, is the seat of the first vessels and blood. In the 

 chick the corpuscles appear during the first days of incubation and 

 before the appearance of a heart. At the end of the first day v , sur- 

 rounding the early embryo there appears a circular, vascular area 

 made up of cords of cells in which are developed the first evidences 

 of the vessels and corpuscles. The corpuscles appear in groups 

 within this branched network of mesoblastic cells, where they form 

 the "blood-islands" of Pander. Presently the cords of mesoblastic 

 cells which compose this network begin to become vacuolated and 

 hollowed out to constitute a system of branching canals, at the same 

 time that their cells acquire the endothelial type. The small, 

 nucleated masses of protoplasm, known as the "blood-islands/' 

 undergo disintegration, whereby their nuclei are set free soon, to 

 collect around themselves a thin envelope of protoplasm. These 

 constitute the primitive red corpuscles, and are the only bodies con- 

 tained within the blood during the first month. In the meantime 

 they have been acquiring a reddish hue, which marks the advent of 

 the haemoglobin. As the canals become extended and branched 

 eventually to connect with the heart as its system of vessels, there 

 appears within them a fluid into which are emptied the red cor- 

 puscles. Thus is completed the circulation. According to Klein, 

 the nuclei of the protoplasmic vessel-walls multiply to form new 



