54 CHARLES R. STOCKARD 



Figure 44 accompanying figure 43 shows a striking contrast 

 in the contents of the cardinal vein. This section is through 

 a more posterior region of the same embryo. The vascular 

 endothelium is here also well differentiated and the vessel is 

 completely packed with early erythroblast some of which are 

 still dividing. None of these erythroblasts, however, have been 

 derived from the vascular endothelium and were actually pres- 

 ent before the endothelium was differentiated. 



The only source of intra-embryonic blood is from the blood 

 anlage which is contained within the intermediate cell mass, 

 as a rule. But in the splitting away of this mass from between 

 the lateral plate and the somites, it is, of course, conceivable 

 that some future blood forming cells might be left either in the 

 somitic portion or the lateral plate portion. In such cases all 

 those organs arising from regions which had been in contact 

 with the intermediate cell mass either medially or laterally might 

 be contaminated with blood forming cells. If the separation of 

 the blood cell anlage takes place in a clean and complete man- 

 ner in the indi\'idual embryo, then I believe the statement is 

 true that all the intraembryonic blood will be contained in the 

 intermediate cell mass or cardial veins which amount to the same 

 thing, 



6. The origin of lymphocytes and leucocytes or so-called white 



blood corpuscles 



We may now turn to a consideration of the origin of lympho- 

 cytes and leucocytes or cells other than red blood corpuscles. 

 Many authors have claimed from observations on various em- 

 bryos that these cells are entn-ely distinct in their origin from the 

 origin of the red blood corpuscles. Both types, however, arise 

 from the same germ layer or mesench3"me. It has been repeat- 

 edly pointed out and seems to be thoroughly substantiated by 

 fact that the lymphocytes and leucocytes in their first appearance 

 are always interstitial in position and are only later contained 

 within the vessels. Whereas, the erythi'oblasts are in\'ariably 

 formed or divided off into the vessels. In other words, the red 



