324 Thos. H. Montgomery. 



are to be seen in Figs. 84-86. At its anterior end, which will become 

 the cephalic artery of the adult, it is bounded by the mesial walls of 

 the rostral mesoblast sacs (Fig. 87a). In the embryo at reversion 

 the pairs of ostia are more numerous than in the adult — about seven 

 or eight in number. An earlier and a later stage in the heart develop- 

 ment are shown in Figs. 88 and 87d, respectively, both being portions 

 of transverse sections of the dorsal region. In Fig. 88 right and 

 left coelomic sacs (Coel.), each with somatic {So. Mes.) and splanch- 

 nic layer (Sp. Mes.) have approximated, and in the midline between 

 them is an archicoelic space, the heart (//), bounded ventrally by 

 yolk (Vit.). This early heart space contains blood cells (Bl.) ; and 

 the splanchnic mesoblast on each side of it is thickened to make the 

 beginning of the walls of the heart tube. The later condition is shown 

 in Fig. 87d; the heart cavity (H.) is now completely enclosed by 

 splanchnic mesoblast, this being the heart wall, while the coelomic 

 space on either side of it is the pericardial cavity. There is no doubt 

 that at this stage the heart cavity is archicoelic, its wall of splanchnic 

 mesoblast, and the pericardial cavity coelomic in origin. Within the 

 heart lie two kinds of blood cells ; smaller cells, the origin of which I 

 have not traced, though there seems to be no evidence of mesoblastic 

 origin, and the large cells with chromidial nets whose history we 

 have learned. Most of these larger cells are now, as before, archicoelic 

 in position, placed between the yolk and the splanchnic mesoblast or 

 between the ectoblast and the somatic mesoblast (Fig. 88). But 

 occasionally they are found within the coelom, as is the case with 

 the most right-hand one of Fig. 87d; this is not surprising, for the 

 mesoblast is discontinuous at many points, and at any one of them a 

 blood cell could pass from the archicoel into the coelom. These blood 

 cells are for the most part dorso-median, within the heart, but many 

 lie right and left of it (Figs. 88, 87d), at the anterior end of the 

 heart there is a crowded mass of them (Fig. 87a) and others also in 

 more lateral positions (Figs. 87a, b, 89-91a) between ectoblast and 

 yolk. But anterior to the caudal lobe there are none in ventral posi- 

 tion and relatively few far lateral from the dorso-median line, hence 

 the reversion process has translocated most of them dorsad. The pos- 

 terior end of the heart is drawn in Fig. 91b, where some of the blood 



