6o HOWELL. [Vol. IV. 



It is permissible, perhaps, to suppose that the large oval 

 corpuscles represent the form of the red corpuscle character- 

 istic of the ancestors of the mammalia, and to speak of them 

 therefore as ancestral corpuscles, while the smaller circular 

 corpuscles of the usual size of the nucleated red corpuscles of 

 the mammalia exhibit a modification of this ancestral form 

 which has become characteristic of the blood of most of the 

 mammalia. These latter corpuscles under normal conditions 

 lose their nuclei and become changed to the biconcave red 

 corpuscles of the circulating blood, the transition, in the young 

 embryo, taking place in the blood itself. It is not probable 

 that there is any essential difference in the way in which the 

 two forms of red corpuscles are produced ; but it is possible 

 that the large (ancestral) form represents an entire embryonic 

 cell, in which haemoglobin has become developed, while the 

 true mammalian form arises from similar cells after they have 

 broken up by karyokinesis into smaller daughter-cells, in each 

 of which the nucleus is larger relatively to the size of the whole 

 cell. A similar difference in the red corpuscles of the very 

 young embryo has been recorded by Erb (4). He states that, 

 in the blood of two young pig embryos (i in. long) he found 

 some nucleated red corpuscles of great size and elliptical form, 

 having a superficial resemblance to the red corpuscles of the 

 frog. Most of the red corpuscles had nuclei, and those that 

 had not were always the smaller variety, similar in outline to 

 the corpuscles of the adult animal. 



The nuclei of the nucleated red corpuscles of the young 

 embryo (except the larger variety) are lost while in the circula- 

 tion, and the presumption is that this fate is met by all the 

 truly mammalian red corpuscles. As the embryo grows older 

 and the production of new corpuscles becomes localized in 

 different organs, — liver, spleen, and marrow, — more and more 

 of the early history of the corpuscle is passed over while still 

 in the blood-forming organ, and more and more of the red 

 corpuscles are sent into the blood stream in the non-nucleated 

 stage. After birth, and throughout adult life under normal 

 conditions, we find only non-nucleated red corpuscles in the 

 circulating blood. The nucleated period in their life-history 

 has passed while they were in the blood-forming organ (the 

 red marrow). 



