ERYTHROPOIESIS IN YOLK-SAC OF PIG EMBRYO 283 



e. The blood cells 



1) Terminology. Four distinct types of cells may be recog- 

 nized: 1) The haemoblasts, or blood mother-cells. These cor- 

 respond with the priinitive 'lymphocytes' of Maximow (16), 

 and the 'mesamoeboid cells' of Minot (19). 2) The erythro- 

 blasts, corresponding with the 'ichthyoid' blood cell of Minot, 

 and in part with the 'megaloblast' of Maximow. 3) The 

 normoblasts, corresponding with the 'sauroid' cell of Minot. 

 The last two may be designated inclusively as erythrocytes. 

 4) The giant cells, both megakaryocytes and polykaryocytes. 



The majority of the blood cells can be classified under one or 

 the other of the above heads. However, between typical primi- 

 tive haemoblasts and erj^throblasts, and between the latter and 

 normoblasts, as also between haemoblasts and giant cells, com- 

 plete series of transition forms occur. 



Up to the 15 mm. stage no cell is present that can be certainly 

 identified as a leucocyte. The haemoblasts are structurally 

 very similar to the lymphocytes of the adult, and if they are 

 indeed in part at least, functionally identical, as claimed by Maxi- 

 mow in support of the monophyletic theory of haemopoiesis, 

 they may be properly designated 'lymphocytes.' 



2) Haemoblasts. This terminology implies that the cell 

 designated 'haemoblast' is the common mother-cell of both leu- 

 cocytes and erythrocytes. No evidence, besides its very close 

 similarity to a lymphocyte, accrues from this study to indicate 

 that the cell in question is also a leucocyte progenitor. It may 

 be noted, however, that this cell would apparently have to under- 

 go less differentiation in becoming a mononuclear, or even a 

 polymorphonuclear, leucocyte, than in becoming an erythro- 

 plastid. Moreover, there is now a very considerable body of 

 embryologic data to show that this cell in certain mammals 

 (rabbit, Maximow (16); birds, Dantschakoff (5); reptiles, Dant- 

 schakoff (6), and Jordan and Flippin (14); and selachii and 

 amphibia, Maximow (17)) is indeed the parent cell of both red 

 and white blood corpuscles. Thus while the haemogenic proc- 



