ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 567 



Ctenophores and Tunioates there is a very strict germinal localization, 

 but they mark one extreme. 



Behind deutoplasmic material, with which Brachet includes mito- 

 chondria and all that the protoplasm has made, there are probably 

 quantitative regional differences in the protoplasm proper. There are 

 probablv zones of different concentration, yet qualitatively uniform, 

 differences of surface tension and capillarity, differences of rate of 

 reaction. Different kinds of eggs may differ in their physical topography, 

 as crystals in their faces and angles. 



If a part of an ovum is to serve as the equivalent of a whole, a 

 restoration of the typical topography must first occur, and the propor- 

 tions of material must be approximately re-established. Does the 

 nucleus during oogenesis influence localization, or do hormones in the 

 parental environment play a part ? Or is the topography aboriginal — 

 the specific character ? The author holds to the idea of the ovum as 

 specifically heterotropic. In the course of development the initial 

 difference's between the blastomeres become accentuated and complicated 

 by the different relations established ; the specific topography is lost in 

 varying degrees ; the " total potentiality " of the cells dwindles as their 

 differentiation becomes accentuated. 



Yolk-sac of Pig Embryo.* — H. E. Jordan has made a microscopic 

 study of the yolk-sac of the pig embryo to obtain further data regarding 

 the earliest stages in blood-cell origin and development in mammals. A. 

 preliminary report of the work was previously published ; the i)resent 

 paper contains full details of more extended observations, and some 

 revised interpretations. The yolk-sac. attains its highest stage of pro- 

 gressive histologic differentiation in pig embryos of about 10 mm. in 

 length. Tbis is true both of the endoderm and the angioblast. The 

 endodermal cells are characterized chiefly by abundant presecretion 

 filaments, apparently identical with the cytoplasmic threads found in the 

 cells of the liver and of the mesonephric tubules. Angioblast arises 

 from mesenchyma. The mesothelium of the yolk-sac of pig embryos 

 between 5 and 12 mm. does not produce hsemoblasts, nor is there 

 satisfactory evidence that the mesothelium of the body-stalk and chorion 

 function to this end. The mesenchyma may differentiate directly into 

 endothelium or into haemoblasts. These last arise extensively at the 

 10 mm. stage from the endothelium of the yolk-sac blood-vessels. The 

 endothelia of the hepatic sinusoids and mesonephric glomeruli of this 

 stage also show extensive hgemopoieiic capacity. Giant-cells, both mono- 

 and polynuclear, are abundantly present in the yolk-sac only at about 

 the 10 mm. stage of development. They may arise from endothelium 

 or directly from haemoblasts. They are giant htemoblasts, and 

 apparently function as multiple erythroblasts in which normoblasts 

 differentiate intracellularly. The several stages in haemopoiesis, repre- 

 sented successively by haemoblasts, erythroblasts and normoblasts, with 

 transition stages, are abundantly present in the yolk-sac of embryos 

 from 5-15 mm. 



* Amer. Journ. Anat., xix. (1916) pp. 277-304 (2 pis.). 



