8 S. F. CLARKE. 



surface of the body is now covered with cilia, by aid of which it 

 keeps up a horizontal rotatory motion upon its axis. 



In a ventral view of an embryo, at about this stage, we would 

 also notice this change in form, and we would see that tiie anal 

 end of the medullary folds extend farther around on the ventral 

 side than the cephalic end. Plate 2, Figure 9. 



A constriction now makes its appearance in the throat region, 

 thus defining the head from the body. At the same time, the 

 remainder of the region of the neural canal becomes more dis- 

 tinctly outlined ; a swelling or slightly oval prominence appears 

 on each side of the head, the first external indications of the optic 

 vesicles. Plate 2, Figure 10. In a dorsal view, a line running 

 alona: the centre of the neural canal indicates the line of union of 

 the medullary folds. Plate 2, Figure 11. In a ventral view of 

 the same are seen both the optic vesicles, the ridge of the medul- 

 lary fold between them, the constriction of the neck and the anus 

 at the posterior end of the neural tube. Plate 2, Figure 12. The 

 embryo having reached this stage, a second groove or furrow 

 appears in the neck-region, so that the throat is now marked off 

 both from the head and from the body. The anterior end of the 

 neural canal or head now bends forward and downward upon 

 itself, so that, by this cranial flexure, the fore-brain, with its optic 

 vesicles, no longer occupies the anterior end of the longitudinal 

 axis. The head has also changed in shape, having no longer a 

 simple rounded outline. In the anterior portion of the neural 

 canal there appear a few transverse swellings, the first indications 

 of the protovertebrae. Plate 2, Figure 13. These latter soon 

 increase in number, additional ones making their appearance pos- 

 teriorly ; the neck region becomes larger ; the optic vesicles be- 

 come more rounded and more prominent. There is next seen pro- 

 jecting from the sides of the neck behind and above the prominence 

 of the optic vesicles, a pair of lobes, one on each side ; from these 

 lobes are to be developed the branchiae. A little posterior to the 

 branchial lobes, there has also appeared another pair of lobes; 

 from these will be developed the anterior limbs. The optic vesi- 

 cles are still more prominent, and the protovertebrae now appear 

 in a side view to be somewhat removed from the outer edge of the 

 neural canal towards its centre; they are also larger. 



Development now progresses at both extremities, and the entire 

 body increases rapidly in size. The head is still farther separated 



