lymphocytes appeared only sporadically in the 

 circulating blood, even up through hatching. 



It is apparent that additional studies on the 

 nuniljer of cellular elements in the circulating 

 blood of the embiyo are needed to give us a more 

 firmly established baseline. 



DESCRIPTION OF THE CELLS 



A great deal can be learned concerning blood- 

 cell lineage and morphogenesis from a study of 

 the circulating blood in embryos at various ages. 

 When this is supplemented by studies on hema- 

 topoietic tissues, spleen, bone marrow, and thy- 

 mus, a fairly complete picture can be obtained of 

 the interrelationship of cells. Without this back- 

 ground the various circulating blood and hema- 

 topoietic tissues of the adult fall into numerous 

 unrelated series of cells. 



Pritnary erythrocytes 



Satisfactory preparation of the primary eryth- 

 rocytes at 2 days incubation recjuires careful at- 

 tention to technic. Most of the early smears 

 gave shrunken cells that carried numerous proto- 

 plasmic processes around their periphery. They 

 looked like cells A, 11-15, of figure 224 

 but most of them were worse. Any contamina- 

 tion of the pipette with saline used to float the 

 embryo or with albumen or yolk would produce 

 this effect, and if there was any delay in getting 

 the cells on the slide, spreading them out into a 

 thin layer, and drying them, they would be 

 greatly distorted. 



At all early incubation stages, satisfactory 

 ])reparation was found to depend greatly on 

 speed in making the smears. With a few^ refine- 

 ments in technic, blood could be taken from the 

 dorsal aorta of the embryo as soon as circulation 

 begins, or it could be taken from vitelline ves- 

 sels without contaminating it with other fluids. 



A considerable difference in degree of dif- 

 ferentiation exists between A and B of figure 224; 

 yet there is only 1 hour of difference in incubation 

 age. It is suspected that the development of the 

 embryo represented by A was retarded. Prac- 

 tically all the cells at this age, according to the 

 terminology of Doan, Cunningham, and Sabin 



(1925), are megaloblasts and are destined to 

 produce erythrocytes. 



The structure of the early primary erythroblast 

 is shown in cells 7-70 of figure 224: A. Probably 

 the least differentiated cell is 2, which has a 

 cytoplasm of uniform texture cjuite different 

 from that of die other cells. This cell was added 

 to the drawing from another part of the same 

 slide, as were also 5, 11-14, and 17. Con- 

 sideration of the significance of this cell is helped 

 by looking ahead to figure 225. Within less 

 than 24 hours of additional inculcation these will 

 have differentiated in the blood into two distinct 

 cell lines — the dominant primary erythrocytes 

 and the embryo thrombocytes. The latter are 

 said to come from the same precursor cells as the 

 erythi'ocytes. Therefore, it is quite possible 

 that these are early embryonic thromboblasts. 

 Had Ilalph's jjenzidine technic been applied 

 (p. 231 ), the point in cjuestion could have been 

 settled easily; with this technic the cytoplasm of 

 primary erythrocyte cells gives a positive yellow 

 color when hemoglobin is present, while the cyto- 

 plasm of the thromliocytes is negative. The 

 appearance of the cell differs also from that of 

 the early primary erythroblast shown in figure 

 233. The latter has a cytosome filled with mito- 

 chondrial spaces " surrounded by granular cyto- 

 plasm that has a type of texture characteristic of 

 primary erythroblasts. The primary erythro- 

 blasts in which tiiis texture is found include those 

 undergoing mitosis (fig. 224, A 16. B 21, and 

 22:'- 



In the cytosome of the primary erythroblast 

 are three types of spherical bodies. One of these 

 is probalily an artifact. It appears in the cells 

 of figure 224 A as clear spherical spaces. The 

 spaces may be small as in 2 and 5 or large as in 

 15 and 16. Although they appear to lie within 

 the luicleus this is an artifact. A vacuole or other 



' Takagi ( 1931 ) showed that rod and filamentous mito- 

 chondria are present in the hlood cells of the yolk sac of the 

 early embryo, but he was not concerned with the specific 

 problem of the identity of mitochondria with cytoplasmic 

 spaces. Jones (19471, who was interested in the structure of 

 primitive erythroblasts. concluded that (p. 317): 



"Light areas in basophilic cytoplasm previously described 

 as hyaloplasjn or paraplasm represent, for the most part, the 

 negative images of underlying mitochondria." 



" Takagi (1932) studied the distribution of chondriosomes in 

 dividing blood cells. He used the yolk sac of chick embryos 

 and made his observations on material sectioned in paraffin. 

 At the metaphase. the chondriosomes were groupeil around 

 the poles of the dividing cell: none were located either in or 

 on the spindle. During anaphase and telophase, they moved 

 toward the region of the constricting cell walls. 



115 



