42 JOHNS HOPKINS UNIVEESITY MORPHOLOGICAL MONOGRAPHS. 



the notochord and the pharynx. It quickly becomes a large hollow 

 vesicle, Plate XVIII, Figs. 1, 2, 3 and 4, /, which runs vertically behind 

 the pharynx. On the side next the pharynx the heart, e, is formed as a 

 vertical groove or fold in the wall of the pericardium, which ultimately 

 becomes converted into a tube by the union of its edges, as shown at c in 

 Plate XIX; the union first taking place in the middle, and extending 

 towards the ends, which are permanently open and communicate with 

 the space of the body cavity in which the blood circulates. As the heart 

 becomes shut in, the follicle cells which lie between it and the wall of the 

 pharynx, as shown in Plate XVII, become folded into it, as shown at e in 

 Plate XIX, Figs. 8 and 9, and they then become vacuolated, and ulti- 

 mately disappear. At the stage shown in Plate XIX the heart has 

 essentially its adult form, and the series of sections illustrates its position 

 and anatomical structure with sufficient clearness. Figure 1 is near the 

 top of the heart, and shows its opening into the body cavity, which is 

 shown in Fig. 6 more enlarged and filled with blood corpuscles, which are 

 formed in the body cavity and are drawn into the heart by its pulsations. 



Fig. 2, which is shown more magnified in Fig. 8, and Fig. 3, which is 

 shown more magnified in Fig. 9, show the structure of the middle region 

 of the heart, while Fig. 5 shows its inferior opening into the body cavity. 

 The heart changes its position with the growth of the embryo. It is at 

 first behind the pharynx, as shown in Plate XLI, Fig. 2, /, but as the 

 pharynx elongates it pushes the heart down and grows over it, as shown 

 in Plate III, Fig. 4, e and /, where the heart is in its adult position under 

 the posterior end of the pharynx. 



I have not been able to trace the origin of the mesoderm, and have 

 first found it at the stage shown in Plate XLII, Fig. 8. At the lower part 

 of this figure a number of small cells, with deeply stained nuclei, much 

 smaller than those of the follicle cells, are shown in the body cavity, y, 

 outside the visceral follicle cells, and other similar cells, mes, are shown 

 arranged in an epithelium on the surface of the visceral cells, 8. These 

 are mesoderm cells, some of which are shown more magnified at A in 

 Fig. 9, and in Plate XLII, Fig. 7. Their position seems to indicate that 

 they are derived from the endodermal blastomeres of the pharynx. 



SECTION 6. The Degeneration of the Follicle. 



We must now turn back from the history of the embryonic cells and 

 their derivatives and trace the fate of the follicle cells. At a very early 



