ALIMENTARY CANAL IN LEPIDOSIEEN AND PROTOPTEEDS. 495 
from the external epiblast, gradually transforming the 
original ^'endoderm” yolk-laden cells into ectoderm like 
itself/’ Apart from the difficulty of believing in the possi- 
bilitj' of a layer of soft protoplasmic ectoderm cells growing 
inwards and pushing aside compact masses of yolk-granules 
without even producing any signs of mechanical disturbance 
of the tissues, the study of carefully prepared celloidin 
sections is, I think, sufficient to convince anyone that it is 
really a process of conversion in situ which is taking place. 
In such sections there is frequently visible a quite broad zone 
of transition in which the richly protoplasmic '^ectoderm” 
cells pass by imperceptible gradations into the typical yolk- 
cells, there being no trace of the absolutely sharp boundary 
which must be present were the ingrowth hypothesis cor- 
rect. 
To the main pai-t of the buccal cavity, which arises by 
cytolysis in the midst of an originally solid mass of yolk- 
cells, and the walls of which give rise to the teeth, as has 
been described in Part III, there becomes added in later 
stages of development the antero-lateral part of the definitive 
buccal cavity, in the roof of which are situated the narial 
openings. This additional part of the buccal cavity arises in 
ontogeny in the same kind of way as the whole cavity does 
in Polypterus, i.e. by the walling in of a space on the 
lower side of the head, through the development of the upper 
lip and the forward growth of the lower jaw. The mode of 
development of this part of the buccal cavity is made clear 
by text-fig. () A — F. In stages XXXI and XXXII of Protop- 
terus (text-fig. 6 A and b) the position of the front end of 
the alimentary canal is marked out in a ventral view of the 
larva by a transverse line — the line of junction of the yolk- 
laden enteric cells with the ectoderm. Some little distance in 
front of the outer end of this line upon each side is seen a 
dimple, which marks the olfactory rudiment. In a specimen 
rather younger than stage XXXIV in its general features the 
olfactory dimple (text-fig. 6 c) is seen to have become elon- 
gated in an oblique direction, so that its long axis passes from 
