§4 HOWELL. [Vol. IV. 



future nucleated red corpuscles. These are not naked nuclei, 

 but are surrounded by a very thin envelope of colorless proto- 

 plasm. The protoplasmic layer becomes enlarged, and small 

 granules are constricted off from the nucleus, and set free in 

 the cell. In some way these nuclear granules start the forma- 

 tion of haemoglobin, either because they contain the necessary 

 iron or because they act as a sort of haemoglobin ferment. As 

 the haemoglobin develops, the granules disappear, and the nu- 

 cleus becomes smaller. In the mammals the nucleus becomes 

 entirely absorbed in the process, so that the fully formed mam- 

 malian corpuscle is non-nucleated. 



If we attempt to sum up the facts with reference to the de- 

 velopment of the red corpuscles which seem to be fairly well 

 established, we will be obliged, as one can readily see from the 

 foresfoine review, to confine ourselves to a few fundamental 

 points. In the first place, it is perfectly well proved that 

 during extra-uterine life the red corpuscles are developed in 

 the red marrow of the bones. Whether or not the spleen and 

 the lymph glands participate in this function is not definitely 

 determined. In the second place, it is generally admitted that 

 the red corpuscle is first a nucleated cell, and that it loses its 

 nucleus in the marrow or other blood-forming organ. Whether 

 the nucleus is lost by extrusion or disappears within the cell 

 by absorption is not settled; but the majority of writers cer- 

 tainly favor the latter view. In the third place, it is pretty 

 conclusively shown that the nucleated red corpuscle is derived 

 from a colorless cell — erythroblast, to use Lowit's term — 

 which is formed in the marrow. The origin of this cell is the 

 point about which, perhaps, there is least agreement. Finally, 

 none of the recent work supports the theory that the red cor- 

 puscles are derived from the white corpuscles (leucocytes) of 

 the circulating blood, so that this time-honored theory must be 

 definitely abandoned. 



Experimental Work. 



My own work has been confined almost entirely to one 

 mammal, the cat, partly because there was not sufficient time 

 to make a complete series of parallel experiments and observa- 

 tions upon other animals, and partly because, by confining 

 the work to a single mammal, a thorough familiarity with the 



