Hemopoiesis in the Mongoose Embryo. 295 



The study of the origin of the blood-cells is best approached by a 

 classification of the different types of cells found free in the blood- 

 vessels, both yolk-sac and intraembryonic. The preponderating type 

 of cell is the erythroblast with pale homogeneous cytoplasm and a 

 spherical vesicular nucleus. The nucleus generally contains one or 

 several nucleoli and a delicate, more or less granular chromatic retic- 

 utum (fig. 2). Many of these cells are in mitosis; an occasional cell 

 shows a nuclear condition suggestive of amitosis (fig. 2). The homo- 

 geneous cytoplasm has a slightly acidophilic staining reaction; these 

 cells correspond to those of the pig embryo, which, in Giemsa-stained 

 material, have a grayish-pink color. An occasional cell has a slightly 

 larger size than the average of the eythroblasts and is characterized by 

 a finely granular cytoplasm, slightly acidophilic (fig. 126). This is a 

 still younger erythroblast, and corresponds to Maximow's megaloblast 

 type. The granules are more probably the initial hemoglobin content. 

 These cells are transition forms between basophilic hemoblasts and 

 acidophilic erythroblasts. A few cells occur which are characterized 

 by a denser, deeper-staining nucleus and a more highly acidophilic 

 cytoplasm; these represent more differentiated erythroblasts and may 

 be called normoblasts; no non-nucleated erythroplastids are yet present. 



Hemoblasts also appear in considerable numbers, more abundantly 

 in the yolk-sac vessels (fig. 1, a and 6). These are characterized by a 

 relatively large granular and vesicular nucleus, usually with several 

 large chromatic nucleoli and a relatively thin shell of basophilic, ap- 

 parently homogeneous, cytoplasm. In this material not a single 

 mitotically dividing hemoblast was seen. In the pig embryo also a 

 mitotically dividing hemoblast was an extremely rare occurrence. In 

 both instances, however, nuclear amitotic phenomena are abundant. 

 The initial stage is characterized by a kidney-shaped nucleus. The 

 hemoblast may take very irregular shapes (see figure 16) indicating 

 amoeboid activity. Figure 1, c and d, shows two late differentiation 

 stages of a hemoblast; the nucleus stains less intensely, and the cyto- 

 plasm has become less basophilic. Figure Id is binucleated, probably 

 the result of a direct division of the nucleus. Trinucleated hemoblasts 

 also are of frequent occurrence. 



Giant cells are frequently met with. These differ from the hemo- 

 blasts apparently only in their greater size and the irregular character 

 (fig. le) or the multiple condition of the nucleus. The size relation 

 between nucleus and cytoplasm is slightly altered in favor of the latter. 

 The cytoplasm is basophilic and the nucleus generally stains as intensely 

 as that of the mononucleated hemoblast. The giant cell is most prob- 

 ably a hypertrophied hemoblast. Its multinucleated condition re- 

 sults from amitotic division of the hemoblast nucleus. The multi- 

 nucleated forms are much less abundant in the 5-mm. mongoose embryo 

 than in the 10-mm. pig embryo. Moreover, in the latter they have 



