THE HEART. 135 



ectoderm and endoderm. The diagrams (PI. XIII, figs. 

 14 — 17) will enable the reader to understand at a glance 

 the origin of the various parts of the vascular tracts and their 

 relation to the coelom. 



In Stage g the heart has the form of a tube with thin walls 

 and flattened nuclei ; it lies freely in the pericardial cavity, 

 and cords of cell project from its walls into the latter. These 

 cords, the origin of which I have not been able to make out — 

 they first appear in Stage f (PI. IX, figs. 43, 45, 46, c. c.) 

 wheu the dorsal divisions of the anterior somites are disap- 

 pearing — seem to become transformed in the later stages into 

 a very remarkable tissue. The structui-e of this tissue, which 

 may be called the pericardial network, or reticular tissue of 

 the pericardium, will be best seen by reference to PI. XII, 

 fig. 13, which represents a transverse section through the 

 dorsal part of an embryo shortly before birth. ^ The wall of 

 the heart is prolonged into delicate processes, which are con- 

 tinuous with a network occupying a considerable part of the 

 lateral region of the pericardium. This network contains 

 round nuclei in its nodes, and is continuous with the floor and 

 roof of the pei-icardium. Sometimes the nuclei occur singly, but 

 often they occur in masses (right hand side of fig. 13), which 

 often have the appearance of multinucleated cells lying freely 

 in the pericardium. A careful examination, however, shows 

 that they are nodes of the network already described, and that 

 they only differ from the other nodes in possessing more than 

 ope of the round nuclei. Occasionally an apparently free cell 

 with one nucleus may be seen lying in the spaces of the reti- 

 culum. This reticular tissue is, I think, derived mainly from 

 the strings (c. c.) of the earlier stages, and it persists as the pecu- 

 liar cellular tissue which has been described by Gafifron (No. 24) 

 in the pericardial cavity of the adult. Gafifron compares this 

 tissue to the fat bodies of other Tracheates, a comparison with 



^ The apparent; fusion of the dorsal and ventral walls of the heart to the 

 dorsal and ventral walls of the pericardium in this figure is due to the con- 

 traction of the specimen. The heart at this stage, excepting for the network 

 about to be mentioned, lies quite freely in the pericardium. 



