ch. vi ORIGIN OF THE HEAET AND VESSELS 361 



At a stage with twelve segments this Y-shaped vascular rudi- 

 ment is continued forwards as a couple of strands of cells, lying on 

 each side on the inner surface of the splanchnic mesoderm and 

 apparently derived from it. These are destined to give rise in their 

 hinder portions to the two vitelline veins and in their anterior 

 region to the first rudiments of the heart (Fig. 172, A, enc). 



At a stage with fifteen segments the paired strands of cells have 

 assumed a disposition like that shown in Fig. 173, B. They approach 

 one another as the mesoderm extends downwards and presently fuse 

 across the mesial plane (Fig.- 172, B and C), the fused portion being 

 the rudiment of the heart while the two anterior limbs represent 

 the first (mandibular) pair of aortic arches and the two posterior 



Fig. 172. — Ventral portions of transverse sections of young Amphibians to illustrate the 

 development of the heart. (Based on figures by Mollier, 1906.) 



A, J3, D, E, Triton (A twelve segments, B sixteen do., D twenty do., E twenty-six do.); C, Sana, 

 d.mc, dorsal mesocardium ; ect, ectoderm ; cnc, endocardium ; end, endoderm ; mc, myocardium ; 

 rues, mesoderm ; pc t pericardiac cavity ; v.mc, ventral mesocardium. 



limbs the vitelline veins. The heart rudiment is at first extremely 

 short in an antero-posterior direction being much broader than it is 

 long. This is correlated with the shortness of the foregut. As the 

 latter lengthens the heart rudiment keeps pace with it, and becomes 

 elongated (Fig. 173, D). As it does so the tissue within the rudi- 

 ment becomes loosened and takes the form of a syncytial network 

 with wide meshes. 



In the meantime the mesoderm on each side, now containing a 

 wide coelomic (pericardiac) space, has grown down to the mesial 

 plane ventral to the heart, so as to give rise to a ventral meso- 

 cardium which however only persists for a short time (Fig. 172, 

 C and D, v.mc). About this same period fluid begins to collect in 

 the interstices between the cells of the subintestinal strand, with 

 the result that some of the cells in its interior assume a spherical 



