THE PERICARDIUM AND HEART. 77 



32 A, n}. The right and left pericardial vesicles now grow towards 

 each other and unite above the intestine at the two sides of which 

 they formerly lay ; in exactly the same way they unite below the 

 intestine, i.e., ventrally to it (Fig. 32 A-D), the intestine having been 

 previously invested by certain of the mesoderm-cells which were 

 distributed in the primary body-cavity. 



The circumcrescence of the intestine by the pericardial vesicles 

 and the fusion of these latter, as described by ZIEGLER, strikingly 

 recalls the fusion of two primitive segments in the Annelida to form 

 a segmental cavity (Vol. L, p. 290). We have already drawn atten- 

 tion to the relation of the kidneys (nephridia) to the pericardial 

 cavity. The walls of the pericardial vesicles which come into contact 

 and which, in following the comparison, would be the equivalent of 

 the intestinal mesenteries, seem completely to degenerate, so that the 

 cavities of the two pericardial vesicles unite together to form a 

 common cavity. The formation of the heart, which will be described 

 immediately, takes place outside of this space, i.e., outside of the 

 secondary body-cavity and within the primary body-cavity. This 

 also would agree with the condition in the Annelida, where the dorsal 

 vessel arises between the splanchnic layer of the mesoderm and the 

 entoderm, and therefore in the primary body-cavity (Vol. i., p. 291). 



The formation of the heart is introduced by the circumcrescence of 

 the intestine by the pericardial vesicles. The wall of the vesicles 

 which is turned to the intestine yields the wall of the ventricle. 

 This statement made by ZIEGLER must be taken to mean that, 

 from that wall of the vesicle, elements are produced by delamination 

 which yield the heart, while the wall of the pericardial vesicle itself 

 represents the investing peritoneal epithelium (Fig. 32 B and C). 

 The same process would be repeated in the formation of the auricles. 

 These latter had already arisen as the invaginations of the pericardial 

 vesicles described above (Fig. 32 A). These invaginations unite 

 with the opposite wall of the pericardial vesicle and the auricles, 

 which form by the widening of the originally narrow invaginations, 

 fuse with the rudiment of the ventricle (Fig. 32 B-D). At the 

 points of junction, the apertures and valves between the ventricles 

 and the auricles arise. 



The efferent and afferent vessels of the heart (aortae and branchial 

 veins) arise separately from the rudiment of the heart and are no 

 doubt formed by the grouping together of those mesoderm-cells 

 which are derived from the wall of the pericardium, or were already 

 present in the body-cavity, i.e., they originate as cavities between 



