The study of the development of the parts of the head, on account of the crowding of organs which occurs there, always presents greater difficulties to the investigator than that of the remainder of the body. My observations upon it are correspondingly incomplete. I have, however, made out a few points connected with it in reference to some less well-known organs, which I have thought it worth while calling attention to in this preliminary account.
The continuation of the Pleuro-peritoneal Cavity into the Head.
In the earlier part of this paper (p. [86]) I called attention to the extension of the separation between somatopleure and splanchnopleure into the head, forming a space continuous with the pleuro-peritoneal cavity (Pl. 3, fig. 8a, pp); this becomes more marked in the next stage, and, indeed, the pleuro-peritoneal cavity is present for a considerable time in the head before it becomes visible elsewhere. At the time of the appearance of the second visceral cleft it has become for the most part atrophied, but there persist two separated portions of it in front of the first cleft, and also remnants of it less well marked between and behind the two clefts. The visceral clefts necessarily divide it into separate parts.
The two portions in front of the first visceral cleft remain very conspicuous till the appearance of the external gills, and above the hinder one of the two the fifth nerve bifurcates.
These two are shewn as they appear in a surface view in fig. 14, pp. They are in reality somewhat flattened spaces, lined by a mesoblastic epithelium; the epithelium on the inner surface of the space corresponding to the splanchnopleure, and that on the outer to the somatopleure.
I have not followed the history of these later than the time of the appearance of the external gills.
The presence of the pleuro-peritoneal cavity in the head is interesting, as shewing the fundamental similarity between the head and the remainder of the body.
All my sections seem to prove that it is a portion of the epiblastic involution to form the mouth which is pinched off to form the pituitary body, and not a portion of the hypoblast of the throat. Since Götte (Archiv. für Micr. Anat. Bd. IX.) has also found that the same is the case with the Batrachians and Mammalia, I have little doubt it will be found to be universally the case amongst vertebrates.