194 PROFESSOR MARSHALL AND EDWARD J. BLES. 



the hypoblast of the pharyngeal wall nor the splanchnic mesoblast 

 give origin to the cardiac endothelial cells. 



On the other hand, he finds that at the posterior end of the heart 

 the conditions are different ; here the heart lies close against the 

 commencing liver, the walls of which consist at this stage of yolk 

 cells of varying shape and size. The posterior wall of the heart is 

 formed, as already noticed, by the liver itself (cf. Plate XIII, Fig. 2)/ 

 At this part karyokinetic figures are numerous, indicating rapid 

 cell division, and it is from these cells that Schwink believes the 

 endothelial cells of the heart are formed. He therefore assigns to 

 these cells a hypoblastic origin ; agreeing in this respect with Goette 

 and Rabl, though differing from both these writers in regard to the 

 particular region from which they are derived. 



Our own investigations are not altogether conclusive on the point. 

 We find that in tadpoles measuring from 3 mm. to 3|- mm. in length, 

 while the mesoblast in the trunk forms a continuous and well-defined 

 layer surrounding the body, except in the mid-dorsal part, the 

 condition is different in the pharyngeal region. Plate XIII., Fig. 1, 

 represents a transverse section of a 3| mm. tadpole, the section 

 passing through the auditory sacs above and the developing suckers 

 below. The mesoblast consists dorsally of scattered cells, mostly 

 stellate in form ; at the sides of the pharynx the cells are more 

 spherical in shape and more closely compacted ; they are also more 

 or less definitely arranged in outer and inner layers. Towards 

 the ventral surface these layers — somatic, M O, and splanchnic, 

 M P — become more clearly defined, and are separated by a very 

 distinct space, the future pericardial cavity, C A. The two sheets 

 of mesoblast, of the right and left sides of the body, approach very 

 close to each other in the mid-ventral line, but do not quite meet. 

 The two layers — somatic and splanchnic — into which the mesoblast 

 is split on each side differ very markedly. The outer or somatic 

 layer, M O, lies close to the epiblast, and consists of a single layer 

 of flattened cells. The inner or splanchnic layer, which consists of 

 a single stratum of rather large columnar cells, lies close to the 

 hypoblast opposite to the sides of the pharynx ; but on reaching the 

 ventral surface of the pharynx it bends away from the hypoblast 

 rather sharply, and then again turns inwards towards the median 

 plane. The space that is thus left between the hypoblast and the 



