POSITION OF THE RUDIMENTARY HEART. 383 



greater, or body-circulation ; the right half of the heart is 

 the centre of motion for the former, the left half for the 

 latter. (Cf Table XLI.) 



In the human embryo, and in all other Amniota, the 

 heart originally lies far forward on the lower side of the 

 head, as in Fishes it remains permanently near the throat. 

 Afterwards, with the advancing development of the neck 

 and chest, the heart continually moves further back, until 

 at last it is situated in the lower part of the breast between 

 the lungs. At first its position is symmetrical, in the central 

 plane of the body, so that its longitudinal axis corre- 

 sponds with that of the body (Plate lY. Fig. 8). In most 

 Mammals it retains this symmetrical position permanently ; 

 but in the Apes the axis begins to incline obliquely, and to 

 move the apex of the heart to the left side. This inclination 

 is carried furthest in the Man-like Apes; in the Chim- 

 panzee, Gorilla, and Orang, which also resemble Man in 

 this oblique position of the heart. 



The germ-history of all other parts of the vascular system, 

 like that of the heart, point out many and valuable facts re- 

 garding the history of our descent. But as an accurate know- 

 ledc^e of the complex arrangement of the entire vascular system 

 of Man and other Vertebrates is required, in order to follow the 

 matter sufficiently far to make it intelligible, we cannot here 

 enter into any further detail.^^^ Moreover, many important 

 features -in the Ontogeny of the vascular system, especially 

 in regard to the derivation of its various parts from the 

 secondary germ-layers, are as yet very obscure and doubtful. 

 This is true, for example, of the question as to the origin of 

 the coelom-epithelium — that is, of the cell-layer coating the 

 body-cavity. Probably there is an important phylogenetio 

 58 



