ICXPLAXATIOX OF FHirUES 



l-J lliu'm()i)last (It) in liiial stage of separation i'i'(jiii endotlieliun; ; r, ctnlo- 

 tlielial cell. 



14, 15 and 16 Three successive stages in tlie transformation of an endothelial 

 cell into a haemoblast. E, towards entoderm ; l', towards blood vessel. 



17 Nucleus of endothelial cell in phase of amitotic division. Many nuclei 

 also can be seen in mitosis. 



18 Nucleus from mesenchyma. Note the similarity l)etween nuclei of endo- 

 thelium, mesothelium and mesenchyma. 



19 and 20 Two mesothelial cells, .s-, towards surface. Occasional cells can 

 be seen in mitosis. 



21 Haemoblast in mitosis. The spindle is apiiarently tripolar. Such irreg- 

 idar mitoses if sufficiently common would explain the multinuclear haemoblast 

 with nuclei of various sizes. Haemoblasts apparently divide both mitotically 

 and amitotically. 



22 Haemoblast with nuch'us apparently dividing amitotically. 



23 Large binucleated giant cell; the cytoplasm is at an early phase of differ- 

 entiation into the erythroblast type; the nuclei also are in early, but different, 

 stages of differentiation. 



24 Smaller binucleated giant cell (haemoblast); the nuclei are of the typical 

 haemoblast tyi)e. 



25 Giant cell from yolk-sac of 10 mm. pig embryo with four nuclei, which, 

 with their envelo]iing cytoplasm, are at different stages of differentiation. Two 

 of the nuclei have haemoblast characters, one erythroblast and one (.r) early 

 normol)last characters. The cytoplasm also around .r has normoblast char- 

 acteristics. 



26 Binucleated haemoblast (giant cell) in late stage of lu'ocess of direct 

 intracellular differentiation into two normoblasts. 



27 Haemoblast with one long and several shorter stubby i)seudopods. 



2S Endothelial phagocytic cell (perhaps a differentiating haemoblast) 

 having ingested an erythroblast whose nucleus is Tmdergoing karyorrhexis, the 

 cytoplasm appearing normal. 



29 Portion of wall of yolk-sac of 10 mm. pig embryo showing a small blood 

 island. The cells are allot the early haemoblast stage, and closely related periph- 

 erally to the surrounding mesenchyma, from which they have apparently 

 differentiated. One haemoblast is binucleated. E, entoderm, schematically 

 represented; V, blood vessel. 



30 Portion of wall of yolk-sac of 10 mm. pig embryo showing the differenti- 

 ation of a haemoblast {h) from the mesenchyma. 'The nucleus of the definitive 

 haemoblast is still connected through a chromatic nuclear strand with the nucleus 

 of its sister mesenchymal cell. E, entoderm, schematically represented; 1', 

 blood vessel; mes., mesothelium. 



31 Four adjacent entodermal cells to show especially the 'basal' or presecre- 

 tion filaments. 



32 A group of four adjaccul liver cells from the same cmbi'yo, to show the 

 (dose siriiiiai'ity in luiclcar and cvtoi)lasmic struclurc and form between the 

 hepatic and yolk-sac cnnbryonic ei)ithelium. INIany of the hejiatic cells (not 

 here represented) show mitotic figures; amitotic divisions apparently do not 

 yet occur. 



33, 34 and 35 Various ty])es of giant haemoblasts. Figure 35 is typical of a 

 large srroui) of giant cells whose nuclei ])roliferatc amitotically. 



302 



