296 Papers from the Department of Marine Biology. 



attained a much larger size and may contain many more nuclei than 

 in the former. The evidence from the pig embyro suggested that 

 giant cells were multiple erythroblasts, eventually differentiating into 

 definitive erythrocytes. The giant cells of the mongoose include only 

 the earliest stages of those described for the pig, hence no confirm- 

 atory evidence of the above conclusions regarding their erythro- 

 poietic function accrues from a study of these cells ; but the evidence, 

 so far as it goes, accords with that derived from the study of giant 

 cells of the pig, and the tentative conclusion that they are erythro- 

 blasts rather than erythrophages may remain unaltered. 



A careful study of the mesenchyma shows that numerous cells, both 

 singly and in groups, take on nuclear and cytoplasmic hemoblast 

 characteristics, and eventually round up more or less and separate 

 from the parent mesenchyme. Exactly similar conditions were de- 

 scribed and illustrated for the yolk-sac of the 10-mm. pig embryo and 

 need not be further considered. As single cells these hemoblasts may 

 wander into adjacent vessels; or as groups (blood-islands) they may 

 become inclosed in endothelium forming in the surrounding mesen- 

 chyma, to produce an "angiocyst" of the growing vascular net. 

 Similar conditions have frequently been described by various authors 

 (Maximow, Dantschakoff, and others) in sections of the yolk-sac of 

 various forms. Since Stockard has more recently described compar- 

 able processes in the yolk-sac of the living Fundulus embryo, there 

 need remain no further doubt that mesenchyma does actually differ- 

 entiate directly into hemoblasts and into enveloping endothelium. 



The point of controversy now centers on the question whether the 

 endothelium of the vascular net of the yolk-sac and elsewhere can trans- 

 form into hemoblasts ; for in the Fundulus embryo Stockard claims that 

 the endothelium has no hemogenic capacity; and on the basis of this fact 

 he casts doubt upon evidence contributed as a demonstration for blood- 

 cell origin from endothelium in other forms (Schridde, Maximow, Jordan 

 and Flippin, Jordan, Dandy, and others). However, Reagan's (is) 

 more recent findings in Fundulus embryos contradict Stockard's con- 

 clusions on this point. 



Appearances in the yolk-sac of the pig seemed to permit of no escape 

 from the conclusion that endothelium did in fact transform in part 

 into hemoblasts (s). The yolk-sac of the mongoose gives exactly the 

 same evidence. This may be seen by a glance at figure 2a, 26, or 2c. 

 The cell 6 is at the crucial stage of metamorphosis. It is still contin- 

 uous with the endothelium, and directly continuous with the endothe- 

 lial cell/; but it has all the nuclear and cytoplasmic characteristics of 

 a hemoblast (compare with la and 16). Cell c represents a further 

 stage in the same process and is about to separate from the endothe- 

 lium as a mononculeated giant cell. A final stage is represented in 

 figure 3, where a metamorphosed endothelial cell has just become sepa- 



