264 
This cavity in Batrachus consists of a few irregular spaces exten- 
ding from the infundibulum towards the eye, and enclosed by more or 
less definite walls of mesoderm, which are supplied by the oculo- 
motorius, and which give rise to eye muscles. The cavity is evidently 
a rudimentary structure. It is, however, always sufficiently definite 
to be readily recognized. A most interesting phenomenon connected 
with it, is that cells from the ectoderm are proliferated to meet the 
external walls of the cavity (fig. 15 7). The whole structure con- 
sequently resembles in section the gill clefts that lie posterior to it. 
The cavity, of course, has in Batrachus, no connection with the ali- 
mentary canal, and is consequently not a structure similar to the gill- 
elefts; but remembering the origin of the praemandibular cavity from 
the cells of the dorsal wall of the alimentary canal in Selachians, 
and remembering the fusion which takes place between the ventral 
cells of the median portion of its Anlage and the ectoderm, it is 
surely safe to say that the position of the cavity and its relation to 
the ectoderm in Batrachus, tend to confirm the opinion reached from 
the study of Acanthias, namely, that we have here to do with the 
rudiment of a complete head segment, including originally both a 
dorsal musculature, and a ventral outgrowth from the alimentary 
canal — a gill cleft. 
It is interesting to note that OrreL!) in describing the prae- 
mandibular cavities of Anguis fragilis, speaks of them as two lateral 
wings of mesoderm extending from the praechordal plate, and also 
describes and figures just external to these wings, and near the optic 
vesicles a small group of deeply staining cells in connection with the 
ectoderm. In regard to the group he says, ,,Uber seine Bedeutung 
konnte ich nichts Sicheres erschließen“. The cells are homologous 
with those proliferated from the ectoderm in Batrachus to meet the 
walls of the praemandibular cavity. 
Summary. 
a) The alimentary canal extends primarily, beyond the neural 
plate, to the anterior extremity of the embryo. The downward growth 
of the infundibulum severs an anterior portion of the original canal 
from the remaining tract. 
b) The chorda groove extends to the anterior extremity of the 
primitive alimentary canal, although the chorda proper ends in the 
1) A. Orper, „Über Vorderkopfsomiten und die Kopfhöhle von Anguis) 
fragilis‘. Archiv f. mikrosk. Anatomie, Vol. XXXVI. 
