26 F, L. LANDACRE 
a true oral cavity lined entirely by entoderm, but possessing 
a collar of ectoderm, dorsal to its roof and ventral to its floor, 
This curious relation of the ectoderm to the entoderm is to be 
interpreted as a modification of the usual process of producing 
an oral cavity or stomodaeum by the invagination of ectoderm. 
In the urodele the solid column of oral endoderm which abuts 
against the ectoderm cephalad prevents the formation of the 
usual ectodermal «invagination and we have the ingrowth of 
ectoderm to form a collar around a solid column of endoderm, 
which later splits to form an oral cavity lined by endoderm 
instead of by ectoderm as in other vertebrates. The ectoderm 
which in other vertebrates lines the oral cavity is in the urodeles, 
consequently, separated from the oral cavity by endoderm. 
The ectodermal collar is the structure of greatest interest in 
the present discussion. The shape of the mouth in the urodeles 
is indicated during the solid stage of the oral endoderm by the 
great breadth of the endodermal column; consequently, the 
ectodermal collar is also broad and appears in transverse sections 
as broad plates of ectoderm lying dorsal and ventral to the endo- 
dermal column. These plates are curled at the lateral borders 
of the solid endodermal column, bending dorsolaterally and 
ventrolaterally around the lateral borders of the endodermal 
column and projecting slightly between what will be the roof 
and the floor of the oral cavity. These infolded edges cover the 
future lips of the oral cavity laterally with ectoderm. The 
ingrowth of the ectodermic collar begins first in the 6-mm. 
larvae dorsal to the solid endodermic oral collar and later the 
ventral portion of the collar is formed in the same manner. 
Preceding the formation of the ventral portion of the collar, 
there is mesenchyme between the ventral surface of the endo- 
dermal oral column and the ectoderm of the mandible, which is 
of ectodermal origin. This statement is made in spite of the 
fact that occasionally an endodermic cell is found here. Many 
of these can easily be’shown to be walls of blood-vessels or isolated 
blood-cells which retain their endodermic characters, while 
others can be shown to be solid extensions of blood-vessels, and 
it is probable that all of them are concerned with blood-vessel 
