THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 543 



aorta, lies in the attachment of the dorsal mesentery, by means of 

 which the intestine is connected to the vertebral column ; the other 

 trunk, on the contrary, is imbedded in the ventral mesentery, as far, 

 at least, as such a structure is ever established in the Vertebrates ; it 

 is almost completely metamorphosed into the heart. The latter is 

 therefore nothing else than a peculiarly developed part of a main 

 blood-vessel provided with especially strong muscular walls. 



In the first fundament of the heart there are two diiFerent types 

 to be distinguished, one of which is present in Selachians, Ganoids, 

 Amphibia, and Cyclostomes, the other in Bony Fishes and the higher 

 Vertebrates — Reptiles, Birds, and Mammals. 



In the description of the first type I select as an example the 



Tig. 897.— Cross section through the region of the heart of an embryo of S&lamandra maculosa, 



in which the fourth visceral arch is indicated, af ber Eabl. 

 <l, Epithelium of the intestine ; vm, visceral middle layer ; ep, epidermis ; Ih, anterior part of 



the body-cavity (pericardio-thoracic cavity) ; end, endocardium ; p, pericardium ; vlig, meso- 



cardium anterius. 



development of the heart in the Amphibia, concerning which a 

 detailed account has very recently been published by Rabl. 



In Amphibia the heart is established very far forward in the 

 embryonic body, underneath the pharynx or cavity of the head-gut 

 (figs. 297, 298). The embryonic body-cavity {Ih) reaches into this 

 region, and in cross sections appears upon both sides of the median 

 plane as a narrow fissure. The lateral halves of the body-cavity are 

 separated from each other by a ventral mesentery {vhg), by means 

 of which the under surface of the phaiynx is united with the wall 

 of the body. If we examine the ventral mesentery more closely, we 

 observe that in its middle the two mesodermic layers from which it 

 has been developed separate from each other and allow a small 

 cavity (/i) to appear, the primitive cardiac cavity. This is sur- 



