61 



The evidence from the study of this embryo indicates that hemato- 

 poiesis is an unbroken process, inaugurated in the umbilical vesicle 

 and continued along identical lines in the liver. On grounds of simi- 

 larity in size, condition of cytoplasm, and staining reactions, and the 

 absence of extravascular areas of proliferation in the liver, it would 

 seem that the proliferating cells in the hepatic capillaries are the 

 progeny of the lymphocytes and megaloblasts of the yolk sac. This 

 position receives further support from the observation that in the 

 13 mm embryo here studied proliferation among the megaloblasts and 

 normoblasts especially is considerably more active in the liver than in 

 the yolk sac. 



GoODALL (14) in describing blood formation in the sheep notes 

 that the conditions as seen in a 5 cm fetus are identical with those 

 found in the young human embryo. Here the erythroblasts of the liver 

 are said to be the direct descendants of the first megaloblasts (the 

 only cell found in the heart and vessels prior to the 2 cm stage) in 

 the fetal sheep. According to Bonnet (4), in the sheep the endo- 

 thelium of the first blood vessels is particularly active in the formation 

 of normoblasts. Goodall further notes that the "extrusion of the 

 nucleus probably never naturally occurs". This agrees with Schridde's 

 contention for the human embryo, but is at variance with the careful 

 observations of Howell (19) for the cat, and of Maximow for several 

 of the higher mammals. 



Among the types of blood cells above described are found occasional 

 giant-cells. These are of irregular shape, probably amoeboid, with 

 granular pale-staining cytoplasm, and two or more (Fig. 12 R) nuclei. 

 Their origin appears to be from hypertrophying lymphocytes, as noted 

 by Goodall, Maximow and Saxer. The former consider them phago- 

 cytic and decadent bodies. Saxer, however, regards them as cells 



agonial stage. The hemagonium according to Mollier is a large mono- 

 nuclear, deeply basophile cell arising extravascularly from the reticulum 

 ("indifferent material"), which also gives rise to the endothelium of the 

 blood channels and to the connective tissue. It appears similar to the 

 ordinary large mononuclear lymphocyte and may be identical with 

 Maximow's "lymphocyte''. The hemagonia proliferate giving origin to 

 two series of "hemoblasts" (small lymphocytes) which begin to take on 

 hemoglobin, decrease in size, lose their nuclei and become definitive 

 erythrocytes. The hemoblast, order II (small and basophile), is very 

 similar to a lymphocyte, and, while Mollier cannot decide whether the 

 two are identical, he regards it quite probable that external conditions 

 alone may determine whether a hemoblast shall develop into an erythro- 

 cyte or remain a lymphycyte. 



