PLATE 1 



EXTLAXATIOX OP FIGURES 



(Unless otlu'iwisr specified the illustrations are from a single specimen of the 

 10 mm. stage, the magnification 1000, the fixation witli Zenker's fluid, and the 

 stain employed the haematoxylin-eosin combination). 



1 A group of blood cells from one of the larger yolk-sac vessels of a 6 mm. 

 pig embryo (Helly's fixation; Giemsa stain; magnification, 1500 diameters). 

 a) various types (differentiation stages) of haemoblasts; the sparse naked cyto- 

 plasm has a vague irregular granular character and stains intensely blue; the 

 large vesicular nucleus stains a very light blue and contains a delicate, finely 

 granvdar reticulum and one or several spheroidal or irregular nucleoli staining 

 like the chromatic granules, a bluish orange, b) Young erythroblasts ('megalo- 

 blasts'); the nucleus is relatively smaller and the cytoplasm more voluminous 

 than in the smaller younger haemoblasts; the cytoplasm stains a light blue 

 (brownish graj^ or liluish pink) and contains fine, uniform, spherical granules 

 (probably haemoglobin) ; a cell wall is distinct; the still vesicular nucleus contains 

 a coarsely granular reticulum which stains blue; some of these nuclei still contain 

 a nucleolus, c) Older erythroblasts; the nucleus has become still smaller and 

 more chromatic; the homogeneous cytoplasm is relatively more voluminous and 

 now stains pink, d) Normoblast; the nucleus is .small, granular and chromatic ; 

 the cytoplasm stains brilliant red (in Zenker fixed tissue the cytoplasm consists 

 merely of a coarse irregular unstainal)le reticulum enclosed by a robust cell 

 membrane. 1 



2 Narrow jxirtion of wall of yolk-sac including all of its layers. E, entoderm ; 

 the cells contain many presecretion filaments. Between the entoderm and periph- 

 eral mesenchyma is a large blood vessel containing a few blood cells at various 

 stages in the metamoriihosis into a normoblast (e) ; a) endothelial cell; b) haemo- 

 blast; c) binucleated haemoblast with long pseudopod; d) binucleatcd erythro- 

 blast. M, mesothelium; but., basement membrane; cml., endothelium. 



3 A group of developing blood cells from a yolk-sac j^lood vessel, a and b) 

 young haemoblasts; c) haemoblast with nucleus in process of amitotic division; 

 d) binucleated haemoblast; e) binucleated haemoblast in process of cytoplasmic 

 amitotic constriction, a fairly conunon form of cell; f ) erythroblast (Maximow's 

 'megaloblast') ; g) normoblast. 



4 Portion of wall of yolk-sac of 10 nun. pig embryo including mesothelium, 

 endothelium and the intervening mesenchyma. a) endothelial cells from wall 

 of a blood vessel; b) endothelial cell in early stage of separation from wall of 

 blood vessel to become a haemoblast; c) later stage; d) extravascular haemoblast, 

 separating from the endothelium. 



5 Haemoblast at late stage in process of separation from the endothelium. 



6 Haemoblast, of spindle shape, just about to sejjarate from theendothelimn. 



7 Ilninucleated giant cell; large haemoblast. 



8 Trinucleated haemoblast (giant cell) in final stage of separation from the 

 endothelium (e) . 



9 Trinucleated giant cell, immediately after separation from endothelium. 

 Note the lateral Inisal projections, the points of. final separation. One of the 

 uuchM is ap])arcntly undergoing amitosis. 



10 liinucleatetl haemoblast in which on(> of the nuclei and the surrounding 

 cytoplasm have differentiated into a iu)rmol)last. 



11 and 12 Triniideated giant cells. 



300 



