DETAILS OF LATER EMBRYO. 



105 



loosened at several points, giving the wavy contour noted in the figure of the 

 entire embryo. The arrangement of the vessels is clearly shown, and one traces 

 the posterior umbilical veins and the anterior vitelline arteries. At first sight the 

 yolk-sac seemed to be attached anteriorly throughout the length of the heart region. 

 Later examination, however, showed that a single stalk, albeit a very short one, 

 connected the sac with the trunk in a fashion very much as in the young shark. 

 (67. plate VIII, fig. 49''). 



(7) The lateral line sj's- 

 tem of organs is already 

 established. At either side 

 of the eye sensory pits are 

 present and the backward 

 growth of the lateral line 

 could be traced as far as the 

 anal region. 



DETAILS OF THE FOREGOING 

 EMBRYO. 



In the sections, figs. 

 80-83, are shown details of 

 the foregoing embryo. In 

 the first of these (fig. 80), 

 in the eye structures one 

 observes the proportionally 

 enormous size of the lens. 

 Particularly noticeable, 

 also, are the elaborately 

 branching vessels lying be- 

 tween the eye and the mid- 

 brain (t',t'), a symptom again 

 of the embryo's precocious 

 growth. In the following 

 section (fig. 81) the spiracle (s) is seen to be continuous. Sections through an 

 external gill-filament show, even more conspicuously than in shark, the presence of 

 both vein and artery {a, z'). In the same section we observe one of the blood-filled 

 dilatations (d), which have already been commented upon. This appears at or near 

 the end of the gill-filament. 



In fig. 82, a section through the pelvic region just anterior to the anus, we 

 observe on the right side the opening of a segmental duct at sd. Beside it, at ;///, 

 appears a mesonephric tubule. On the opposite side of the body a corresponding 

 tubule, mf, opens directly into the body cavity. One observes in the same section 

 a dilated caudal vein at cv, and above it the caudal artery-. In the section, fig. 83, 

 we observe that the unpaired fins are already well established and that they are 

 made up largely of mesoblast. The caudal vein and artery appear as before, and 

 the section traverses numerous muscle plates. 



Fig. 80. — Transverse section through the eye region of the embryo shown in 

 plate VIII. fig. 49. 



V, t', Branches of anterior internal carotid. 



