ORIGIN OF VASCULAR TISSUES 73 



in figure 27 is a bilateral structure. Figure 25 shows a well- 

 advanced concrescence. Entoderm can readily be distinguished 

 from ectoderm by the darkly staining globules. In fig-ure 26 

 the entodermal layers have met and fused. The entoderm 

 which is to form a portion of the ventral 'body wall' of the mero- 

 plast (morphologically the entoderm which lies next the yolk) 

 has severed its connection with the tubular pharyngeal ento- 

 derm; as there are no endothelial tubes in the immediate region, 

 the opposed splanchnopleuiic mesodermal layers have come 

 directly together. Such endothelium as there is lies ventral to 

 their point of contact. Mechanical conditions may account for 

 the failure of endothelium to form between visceral mesoderm 

 and entoderm at a level dorsal to this place of contact. More 

 anteriorly, in sections not shown in these figures, bilateral endo- 

 thelial tubes fuse mesially to form a single chambered heart 

 (fig. 27). Well-developed dorsal aortae are present. The thick- 

 ened entoderm on the ventral side of figure 26 is not material 

 derived from the area vasculosa. Its thickness has been con- 

 tributed to by the vertically lying fused entoderm which has 

 subsided in consequence of the severing of connection of the 

 latter with the tubular pharynx. The place of incision was 

 within the limits of the area pellucida. Also the entoderm from 

 each side was somewhat thickened as this region contains the 

 'Narbe' caused by each incision. These conditions are not 

 comparable to those of Graper's figure 24 (16). In my figure 

 27 the meroplast has a complete ventral 'body wall' which is 

 composed partly of entoderm. In some sections posterior to 

 this, the duplex nature of the heart is indicated by the duplicity 

 of the endocardium; here it is single. One can here detect the 

 double nature of the myocardium by the small, median, ven- 

 trjilly directed point of fusion of the originally right and left 

 splanchnopleural anlagen. Opposite, close to, and directed to- 

 wards this myocardial protuberance is a sharp projection of the 

 horizontal splanchnic mesoderm (i.e., the mesoderm associated 

 with the 'body wall' entoderm). These two projections repre- 

 sent sectioned folds which are remnants of the ventral mesen- 

 tery of the heart; they have been continuous structures but have 



