186 EMBRYOLOGY. 



itself arise ? Here, too, the uncertainty of our present knowledge is 

 to be emphasised. 



According to the representation of His, to which KOLLIKER also 

 adheres, and which the author himself has made the foundation of 

 his account in the first edition of this Text -book, blood-vessels in the 

 embryo are not independently formed, but take their origin from 

 those already existing in the opaque area. According to His, the 

 germ of the blood and connective substances, originally a peripheral 

 fundament, makes its way from the opaque area at first into the 

 pellucid area, and from there into the body of the embryo itself, 

 and is distributed everywhere in the spaces between the epithelial 

 germ-layers and the products that have arisen by constriction from 

 them. Into the spaces migrate first of all amoeboid cells, which 

 send out in front of them branched processes ; on the heels of these 

 follow endothelial vascular shoots. 



At variance with the teachings of His are noteworthy investiga- 

 tions of recent date, not only the previously mentioned accounts of 

 the manifold origin of the connective substances from the middle 

 germ-layers, but also particularly the more recent observations con- 

 cerning the independent origin of vessels and the endothelial sac of 

 the heart in the body of the embryo itself. (RiicKERT, ZIEGLER, 

 MAYER, RABL, KASTSCHENKO, and others.) 



For Selachian embryos the question, whether the repository of 

 the material for the blood-vessels of the embryo is to be sought 

 exclusively on the nutritive yolk, is, as RUCKERT remarks, to be 

 answered definitely in the negative. The vessels arise in the embryo 

 itself within the territory of the mesenchyme, from cells which 

 are sometimes loosely, sometimes compactly arranged (RiiCKERT, 

 MAYER). 



RUCKERT derives the cells that form the vessels from two different 

 sources, partly from the inner germ-layer of the yolk-wall, partly 

 from the adjoining mesoblast, and their double origin appears to 

 him a natural process of development, in so far as the two layers 

 which bound the first vessels also furnish the material for their walls. 



To the same purport are the accounts concerning the formation 

 of the endothelial sac of the heart. At first it consists of a rather 

 irregular mass of cells, in which there appear separate cavities, that 

 gradually unite to form a single cardiac space. The cell-material 

 of the fundament of the heart is developed in situ (RtiCKERT, ZIEGLER, 

 MAYER, RABL, and of the earlier investigators GOTTE, BALFOUR, 

 HOFFMANN) from the wall of the bounding germ-layers; however, 



