58 HOWELL. [Vol. IV. 



conditions of health or age. On a priori grounds it seems to 

 me that most persons will incline rather to the other view, that 

 the production of the red corpuscles takes place in one way 

 under all conditions of life, though in some cases the process 

 may be accelerated or abbreviated, and in others retarded. In- 

 asmuch, then, as decisive proof that the red corpuscles are 

 formed by two or more different methods is wanting, it is 

 allowable to examine critically the different theories which 

 have been proposed, and to endeavor to discover whether the 

 phenomena they are intended to explain cannot be grouped 

 under a common theory. My own investigations have ex- 

 tended over a period of two years ; and though they are in 

 many respects incomplete, yet in a number of points satisfac- 

 tory conclusions have been reached, and it seems to me, as I 

 shall endeavor to show, that the phenomena which I have ob- 

 served help to some degree at least in reconciling different 

 theories, and indicate that one general plan of formation holds 

 good in all cases. 



Since 1838 it has been known that the red corpuscles of the 

 foetal animal are at one period nucleated cells. This was first 

 shown by Rudolf Wagner (i) for the embryo bat and by E. H. 

 Weber (2) in a human foetus of twelve weeks. Most of the 

 observations made upon these nucleated red corpuscles of foetal 

 blood, their occurrence and their relative numbers at different 

 periods of foetal life, we owe to Kolliker (3) and to Paget. 

 Kolliker found that in a sheep's embryo of three and one-half 

 lines the red corpuscles are all nucleated, and Paget makes the 

 same statement from the human embryo of four lines (fourth 

 week). In a human embryo of three months the nucleated 

 corpuscles in the circulating blood make up from one-sixth 

 to one-eighth of the total number of corpuscles, while at five 

 months they are still quite numerous. In a human foetus of 

 this age (five months) which came under my own observation, 

 and which had been born about five or six hours, and was 

 brought to me still enclosed in the amniotic sac, I found that 

 the majority of the red corpuscles were not nucleated, though 

 nucleated forms were still very abundant. Among the nucle- 

 ated corpuscles some were found with the nucleus fragmented 

 incompletely into a number of pieces, forming what has been 



