1. A solid post-anal gut is formed behind the blastopore (anus), growing out into the tail, and fusing with the undifferentiated tissues at its posterior end. The fusion of hypoblast and epiblast in this region represents the neurenteric canal.
2. In the Frog the post-anal gut is at first hollow, but afterwards becomes solid.
3. The stomodæum and pituitary body are derived from a solid ingrowth of the inner layer of the epiblast. The hind part of this ingrowth fuses with the front wall of the fore-gut, but the perforation to form the actual mouth does not appear till after hatching. The pituitary body grows upwards as a solid cord, and applies itself to the infundibulum in the ordinary manner.
4. From the hind border of the stomodæum proceeds a solid rod of cells, which constitutes the thyroid body, and is developed from the cells of the middle ventral line of the foregut.
5. The development of the peripheral nervous system is preceded by the appearance of a neural ridge, extending along the whole length of the body.
6. The spinal nerves grow out from the neural ridge, and pass downwards between the neural canal and muscle plates.
7. The cranial nerves also grow out from the neural ridge, but are nearer to the surface than the spinal nerves, owing to the absence of muscle plates in the head.
8. When each has attained a certain length it fuses with a thickening of the epiblast, situated some distance above the level of the notochord. (This is the case with the 5th, 7th, and 9th nerves, and probably also with the vagus.)
9. At the point of fusion there is a thickening of the nervetrunk, forming a ganglion, which afterwards recedes from the surface, remaining, however, attached to the sense organ by a nerve.
10. The main trunk of the nerve passes on, and, in the cases of the 7th and 9th nerves, fuses again with the epiblast of the dorsal wall of the corresponding gill-cleft. Later, the nerve becomes detached from the epiblast, and gives off two branches, one behind and one in front of the gill-cleft.
11. The 5th nerve has no such second (ventral) fusion with the epiblast, but divides below its first (dorsal) fusion into two branches, the superior and inferior maxillary.
12. In the Frog a neural ridge is present at an early stage, just after the closure of the neural canal. The facio-auditory nerve grows out of the brain, and it is therefore probable that the other cranial nerves have the same origin.
N.B.--Our figures are diagrammatic in so far that the outlines of the cells were not perfectly apparent in all sections. This appeared to us to be due to bad preservation, as the better the specimens were preserved the more distinct and complete were the cell outlines. It was generally possible to draw them accurately with a camera and Zeiss obj. D, OC. 2. We have therefore represented them throughout as distinct.
The Control of Photo-Pigmentary Responses in Eyeless Catfish