BONE-MARROW 217 



accepted by some, e.g., "Minol,- -;nul still in accord with the rnonophyletic 

 teaching regards the later eml>rv<mie, fetal and adult hemapoietic organs 

 as simply foci of proliferation and differentiation of primordial blood-cells 

 (hemoblasts) originally derived from the angioblast and subsequently 

 carried by the blood stream to these locations (red marrow, etc.) where they 

 persist throughout life. 



We will follow first the several steps in the developmental process of the 

 red element, to which taken together the name 'erythrocyte' may be appro- 

 priately applied. According to Maximow, in the forms studied by him, 

 there are two lines of erythrocytes, a primitive and a definitive line; the 

 primitive line dies out after a brief span, and, since the definitive line 

 diners only in the matter of smaller size of cells, we may confine our atten- 

 tion to that line. 



The several stages in the differentiation process are: (1) megalollast; 

 (2) norm.ollast; (3) erythrollast and (4) enjtliroplastld ('erythrocyte'). 

 All except the last may multiply by mitosis. The megaloblast differs from 

 its progenitor, the hemoblast, in having somewhat more cytoplasm, and a 

 small amount of hemoglobin, which gives it a light reddish-brown color. 

 Its nucleus is relatively large and vesicular and contains a delicate gran- 

 ular network; this stage corresponds to the ichthyoid stage of Minot; a 

 seemingly appropriate terminology, since it calls attention to the similarity 

 between the adult (phylogenetic) stage of a lower form, and the embryonic 

 (ontogenetic) stage of a higher form. The normal/last is a daughter-cell of 

 the megaloblast and is of somewhat smaller size, containing more hemo- 

 globin, with a somewhat denser nucleus, with larger net-knots; this corre- 

 sponds to the sauroid stage of Minot, the significance of which term is the 

 same as explained above for ichthyoid. The erythroblast differs from the 

 normoblast in having an increased amount of hemoglobin, but mainly in 

 its smaller, more compact nucleus. This cell becomes an erythroplastid 

 through loss of nucleus. (It conduces to simplicity and clearness to regard 

 the megaloblast and normoblast as two developmental phases of the erythro- 

 blasts.) The manner by which the nucleus disappears has been much 

 disputed. It has been held to be absorbed (karyolysis) ; Howell has de- 

 scribed its extrusion either as a single or fragmented body (karyorrhexis) 

 in the cat; Emmel (Amer. Jour. Anat., 1<, 2, 1914) has recently de- 

 scribed for pig a process by which the red corpuscle arises through budding, 

 leaving a nucleated remnant. 



Certain erythroblasts may be smaller than the average, others larger; 

 these dimentionally atypical forms are termed respectively microcytes and 

 megalocytes. 



The large lymphocyte of the blood is believed to be derived from the 

 primitive blood-cells by slight differentiation; perhaps it represents a po- 

 tential hemoblast. The small lymphocyte is simply a daughter-cell of a 

 large; a large, a grown small lymphocyte. 



