RED BLOOD CORPUSCLES 73 



masses found within such cells and described by Ranvier as true 

 microcytes are taken by these authors to be minute portions of the 

 included red cells. Fuchs, especially, (see Fig. 77) describes the 

 disintegration not only of the cytoplasm, but of the nuclei of oc- 

 cluded erythroblasts as well. The formation of red blood cells 

 within the " vaso-formative " cells of the mesoblastic tissues, 

 in the later periods of fetal life at least, must be considered as 

 somewhat doubtful. 



With the appearance of the fetal liver, at a very early period, 

 the bulk of the hemopoietic function seems to be transferred to 

 this organ, in the " blood-islands " of which, nucleated erythroblasts 

 are rapidly formed by karyokinesis. The primitive spleen also as- 

 sumes a small portion of the blood forming function. In neither 

 of these organs, however, does this function appear to persist 

 much beyond the term of intra-uterine life. 



The appearance of the red marrow of fione marks the transfer 

 of a steadily increasing portion of the hemogenic function to this 

 tissue. Moreover, bone marrow seems to be the sole tissue in 

 which the function persists throughout adult life. In this tissue 

 the red blood cells are developed by mitosis, occurring within 

 hemoglobin containing erythroblasts. This process is, however, 

 confined to the marrow tissue proper, the red cells, after the dis- 

 appearance of their nucleus, only secondarily gaining admission to 

 the vascular channels* (Swaen et Brachetf). The disappearance 

 of the nucleus of the daughter erythroblasts may possibly be the 

 result of extrusion (Rindfleisch {), or more probably of solution 

 (Kolliker |j). In the latter case the nucleus is said to disappear by 

 karyotysis ; if during this process the chromatin becomes collected 

 into a small compact mass, the nucleus is said to disappear by 

 pylcnosis. 



The more important histological deductions from the foregoing 

 facts may be stated as follows : 



1. All red blood cells are in their primitive condition nucle- 

 ated, viz., erythroblasts, and consequently biconvex rather than 

 biconcave. 



* In this connection the observation of Ascoli (Giorn. d. R. Accad. di Med. 

 di Torino, 1899), that nucleated red blood cells are constantly present in the 

 efferent tibial vein of the dog, is of interest. 



f Arch, de biol., 1902. 



% Arch. f. mik. Anat, 1879. 



j Handbuch der Anat. 



