THE ORGANS OP THE INTERMEDIATE LAYER OR MESENCHYME. 547 



this place the visceral middle layer (ahh) is also raised up somewhat 

 from the entoderm (sw), so that it causes a projection into the body- 

 cavity ( ph). Here there is developed between the two layers a small 

 cavity, which is surrounded by an endothelial membrane (ihh), the 

 primitive cardiac sac. At their first appearance the halves of the 

 heart lie very far apart. They are to be seen both in the very 

 slightly magnified cross section (fig. 300) and also in the surface view 

 of an embryo Rabbit (fig. 302) at the place indicated by h. They 



ahh 



Fig. 301. 



Figs. 300, 301. Cross section through the head of an embryo Rabbit of the same age as that 

 shown in fig. 302. From KOLLIKER. Fig. 301 is a part of fig. 300 more highly magnified. 



Fig. 300. h, h', Fundaments of the heart ; tr, oesophageal groove. 



Fig. 301. J/, Dorsal groove ; mp, medullary plate ; rw, medullary ridge ; h, outer germ-layer ; 

 dd, inner germ-layer ; dd', its chordal thickening ; sp, undivided middle layer ; hp, parietal, 

 dfp, visceral middle layer ; ph, pericardial part of the body-cavity ; ahh, muscular wall of 

 the heart ; ihh, endothelial layer of the heart ; mes, Lateral undivided part of the middle 

 layer ; sw, intestinal fold, from which the ventral wall of the pharynx is formed. 



afterwards move toward each other in the same manner as in the 

 Chick by the infolding of the splanchnopleure, and come to lie on 

 the under side of the head-gut, where they fuse and are temporarily 

 attached above and below by means of a dorsal and ventral mesentery. 

 Concerning the processes of development just sketched the question 

 may be raised : What relation do the paired and the unpaired funda- 

 ments of the heart sustain to each other ? It is to be answered to 

 this, that the unpaired fundament of the heart, which is present in the 

 lower Vertebrates, is to be regarded as the original form. The double 



