No. I.] BLOOD CORPUSCLES. 5-r 



recently been described by Denys (14), as I shall have occasion 

 to mention later on. 



During the second half of intra-uterine life the spleen also 

 takes part in the formation of red corpuscles. This was first 

 made known by Kolliker, and has been confirmed by a number 

 of observers since his time. In the foetal cat the sequence in 

 which the blood-forming organs enter upon their function is as 

 follows. First, the liver; then, as the production of new red 

 corpuscles in the liver becomes diminished, the function is 

 taken up by the spleen. So that at the time of the maximum 

 activity of the spleen the liver takes but little part in the proc- 

 ess. Still later in embryonic life, after the long bones of the 

 limbs have been formed, one finds that the young marrow has 

 begun to produce new red corpuscles, while the activity of 

 the spleen in this respect has suffered a decided diminution. 

 Shortly before birth it is easy to prove that at least three 

 organs are taking part in the formation of red corpuscles, — 

 namely, the liver, the spleen, and the bone marrow, — and even 

 after birth for a certain short time the same is true ; in the cat, 

 for as long as three or four weeks. Later than this, however, 

 nucleated red corpuscles showing signs of active multiplication 

 are found only in the red marrow of the bone. The liver cer- 

 tainly takes no part at all after this time in the formation of 

 red corpuscles; and in the spleen one finds under normal condi- 

 tions no indication of the presence of nucleated red corpuscles. 

 Whether or not the spleen plays any part in the formation 

 of the colorless cells from which the nucleated red corpuscles 

 are afterwards produced will be discussed later. 



While it must be accepted that during embryonic life red 

 corpuscles are made in the three organs mentioned, it seems 

 to me quite certain, also, that they are formed during this 

 period in other parts or organs of the body, — wherever, in fact, 

 developing blood-vessels (veins) are found, — though I have no 

 evidence for this other than the section already described, 

 which passed through the long axis of the posterior limb and 

 showed a developing blood-vessel with its young corpuscles 

 lying in the muscular tissue. In describing the development 

 of the red corpuscles at this period in the life of the animal, — 

 that is, just before and shortly after birth, — mention should be 

 made of the discovery of the vaso-formative cells by Ranvier 



