DEVELOPMENT OF THE BLOOD VESSELS IN THE FKOG. 193 



wards a short distance from the rudiment of the thyroid body. 

 Beneath this groove the hypoblast of the pharynx and the external 

 epiblast are in close contact along the median line, though right and 

 left of this they are separated by mesoblast which has already split 

 into somatic and splanchnic layers. It is immediately behind this 

 region that the heart is formed, and Eabl, who maintains a hypo- 

 blasts origin for the cardiac endothelial cells, is inclined to connect 

 these genetically with the down-growing ridge of hypoblast imme- 

 diately in front. 



Kiickert* describes the endothelial cells of the heart and great 

 vessels as arising, in Selachians, partly from hypoblast and partly 

 from mesoblast, the first source of origin preponderating in some 

 regions, the second in others. 



The most recent investigator of this point is Schwink,f who has 

 studied the early development of the heart in both Anura and 

 Urodela, the two groups agreeing in all essential respects. He notes 

 that the cardiac endothelial cells must be derived either, (1) from the 

 splanchnic mesoblast; or (2) from the hypoblast of the floor of 

 the pharynx ; or (3) from the hypoblast further back, in the region 

 of the liver. In attempting to decide between these, he pays 

 special attention to the forms and characters of the cells, and 

 to the evidence of active division afforded by karyokinetic nuclear 

 figures. The endothelial cells themselves have no constant form : 

 some are spindle-shaped, with two or more processes ; others are 

 spherical and of various sizes ; and all contain numerous yolk 

 granules. On the other hand, the hypoblast of the ventral wall 

 of the pharynx, and the splanchnic mesoblast alike consist of 

 single layers of short columnar cells of definite shape and size, and 

 closely packed side by side. Nuclear figures are not numerous 

 in the cells of these layers, but those that do occur indicate division 

 in the planes of the layers, and not at right angles to these planes, 

 as must be the case if the endothelial cells are derived from one 

 or other of these layers. Schwink accordingly concludes that neither 



* Kiickfrt, M Ueber die Entstehung der Endothelialen Anlagen des Herzens und 

 der erstf:n Gefassstamme bei Selachier Embryonen : " " Biologisches Centralblatt," 

 viii., 1869, pp. 385-399 and 417-430. 



t Schwink, " Debet die Entwickelung des Herzendothelfl dor Ampliibien," "Anato- 

 nrisehex Anzeiger," v., 5 April, 1890, pp. 207-213. 



O 



