278 H. E. JORDAN 



uted to the following debated questions in haemopoiesis: 

 1) Does the angioblast bear any direct genetic relationship to 

 the entoderm? 2) Does the yolk-sac mesothelium produce 

 haemoblasts? 3) Does the mesenchyma differentiate in part 

 into endothelium? 4) Do haemoblasts arise directly from mes- 

 enchyma? 5) Do haemoblasts differentiate from endothelium? 



6) What is the origin and function of the giant cells of the yolk- 

 sac? The first question involves a careful consideration of the 

 structure of the entoderm; which in turn raises the question: 



7) What is the function of the entoderm in yolk-sacs which 

 contain little or no yolk? 



A preliminary report of this study appeared in the Proceedings 

 of the thirty-first session of the American Association of Anato- 

 mists (Anat. Rec, 9:1, '15, pp. 92-97). In the present paper 

 more extensive observations, with illustrations, are recorded. 

 Moreover, a further study of the entoderm compels a reinter- 

 pretation of the cytoplasmic filaments of these cells; my earlier 

 conclusion that they are mitochondrial in nature no longer seems 

 warranted. 



A portion of this investigation was done at the Marine Bio- 

 logical Laboratory, Woods Hole, Massachusetts during the sum- 

 mer of '14. I take this opportunity to gratefully acknowledge 

 my indebtedness to the institution for the privileges of a research 

 room. 



II. MATERIAL AND METHODS 



The material consists of pig embryos ranging in length from 

 5 to 25 mm. Zenker's and Helly's fluids were used for fixation. 

 The stains employed were the Giemsa blood stain, and the 

 haematoxylin and eosin combination. Sacs of stages within the 

 limits specified differ essentially only with respect of relative 

 abundance of the various types of early blood cells. The 10 

 to 15 mm. stages were soon discovered to be most favorable 

 for this study, since here was included in the same sections 

 both earliest and later stages in haemopoiesis. Haemopoietic 

 phenomena seem to be at their height in the yolk-sac of the pig 

 embryo at about the 10 mm. stage of development. 



