112 



HOWELL. [Vol. IV. 



Summary. 



The chief conclusions to which the investigation has led 

 may be briefly summarized in the order in which they are pre- 

 sented in the paper as follows : — 



1. In the very young embryo two forms of red corpus- 

 cles are found, — one large, oval, and always nucleated, re- 

 sembling the corpuscles of the lower vertebrates, and one 

 small, biconcave, circular in outline, and found both nucleated 

 and non-nucleated. The latter are the true mammalian cor- 

 puscles ; the former represent possibly ancestral corpuscles. 

 The true mammalian corpuscles lose their nuclei by extru- 

 sion. 



2. In the first part of embryonic life new red corpuscles are 

 produced in the liver from groups of mesoblastic cells outlining 

 the position of future blood-vessels (veins). The central cells 

 of these cords become red corpuscles, while the peripheral 

 ones form the walls of the veins. Similar developing blood- 

 vessels are found in the embryonic muscular tissue of the 

 posterior limb. It is probable that new red corpuscles are 

 formed in all parts of the body where blood-vessels are being 

 developed. 



3. In the second half of the embryonic life red corpuscles 

 are formed in the liver, the spleen, and the marrow of the 

 bones, the function being most active first in the liver, then in 

 the spleen, and finally in the red marrow. In the cat the liver 

 and spleen lose this function three or four weeks after birth, 

 and henceforward the red marrow alone produces new red cor- 

 puscles. 



4. The white corpuscles (leucocytes) and blood plates do not 

 occur in the circulating blood of young embryos, but make 

 their appearance in later embryonic life. In the human foetus 

 of five months both are present. 



5. In the healthy animal during extra-uterine life the red 

 corpuscles are produced only in the red marrow. They occur 

 first as nucleated cells, the nucleated red corpuscles, found 

 only in the red marrow of the bones. These cells differ in 

 structure with their age, and two extreme types may be distin- 

 guished, — one mature and ready to be converted to a non- 

 nucleated corpuscle, and one immature, as shown by the char- 



