37 
brain comes in contact with the external ectoderm at either side of 
the mesodermic Anlage, and before the two anterior pairs of cavities 
are formed, the mesoderm is severed along the dotted line, and the 
contact of the infundibulum with the ectoderm extends from side to 
side across the axial line of the embryo. 
Fig. 7. Fig. 8. 
Fig. 7. Diagram of the anterior mesoderm before the appearance of the two anterior 
pairs of head cavities. Ch chorda. irf infundibulum. 1 praemandibular Anlage. 
a Anlage of anterior head cavity. 2 and 2’ mandibular cavity. 3 third head cavity 
(VAN WIJHE). 
Fig. 8. Sagittal section after the fusion of the lateral edges of the neural plate, 
Ch chorda. dr brain. ect ectoderm. V trigeminus. J olfactorius. JZ Optic stalk. @ meso- 
derm median to the ,,anterior‘‘ head cavity. x point of fusion of mesoderm a with the 
ectoderm. m point of fusion of median praemandibular mesoderm with the ectoderm. 
ali alimentary canal. 
At the point m fig. 8 a fusion now takes place between the ecto- 
derm, the cells forming the anterior limit of the alimentary canal, and 
the cells median and ventral to the paired praemandibular cavities. 
At the point z, a fusion also takes place between the ectoderm and 
the cells median (a) to the anterior pair of head cavities. These 
points of fusion between the ectoderm and cells once constituting a 
ventral portion of the primitive alimentary canal, seem to me of interest 
as indicating a possible reduction of gill clefts which once belonged 
to the anterior region of the head, but which, like the anterior extre- 
mity of the canal itself, have disappeared as the cranial flexure devel- 
oped. The relations obtaining in this region of the head in embryonic 
Teleosts, where the cranial flexure is less marked, add strength to 
the above induction, in support of which I shall briefly describe 
towards the close of this paper, the formation of the mouth and of 
the praemandibular cavity in Batrachus Tau. 
The median cells, anterior to the point of the notochord soon 
begin to show signs of degeneration, and their place is later occupied 
15* 
