ERYTHROPOIESIS IN YOLK-SAC OF PIG EMBRYO 287 



pink color in Giemsa preparation, correspond to the 'megalo- 

 blast' described by Maximow in the rabbit. 



Occasionally a disintegrating erythroblast may be seen in- 

 gested by an endothelial cell (fig. 28). This observation indi- 

 cates a phagocytic function on the part of the endothehiim of 

 the yolk-sac vessels. An alternative interpretation will be dis- 

 cussed below. 



4) Normoblasts. The normoblasts differ from the erythro- 

 blasts in that they have a smaller more compact and chromatic 

 nucleus, and a more acidophilic cytoplasm (figs. 1 d, 2 e and 3 g). 

 These cells are very uniform in size. In this character of size 

 uniformity they differ markedly from the similar cells in certain 

 lower forms, for example, in turtles. They multiply extensively 

 by the indirect method of cell division. In Giemsa preparations 

 the cytoplasm stains a brilliant red, the coarsely granular 

 nucleus a deep blue. The nucleus frequently has an irregular 

 lobed contour. The chromatin is frequently gathered into sev- 

 eral large and many smaller clumps, the reticulum being delicate 

 and only slightly chromatic. In preparations fixed in Zenker's 

 fluid, the haemoglobin content has become dissolved, and the 

 cytoplasmic area reveals a coarse wide-meshed reticulum, 

 bounded peripherally by a coarse cell membrane (fig. 3 g). 

 By abstriction of the portion of the cytoplasm containing the 

 excentric nucleus, in the manner described by Emmel (7), the 

 erythrocyte becomes an erythroplastid. These stages in plas- 

 tid formation are still extremely rare in 10 mm. embryos. 



5) Giant cells. These cells include a great variety of different 

 forms and sizes. The extremes include: 1) An enormous cell 

 consisting almost wholly of nucleus, the naked cytoplasm con- 

 stituting a mere shell (figs. 33 and 34). The cytoplasm is baso- 

 philic. The vesicular nucleus is generally extensively lobed and 

 contains many large spheroidal and irregular chromatic masses; 

 its nuclear reticulum is wide-meshed, granular, and intensely 

 chromatic. 2) A cell of similarly large size with generally two 

 or three relatively small, spherical, oval or irregular, pale stain- 

 ing, granular nuclei (figs. 23, 24 and 25). The nuclei may 

 contain one or several nucleoli ; and the reticulum is more regular, 



