144 The Cardiac Glands of Mammals 
As regards the supposed continuation of the cesophagus by the ceso- 
phageal groove it is found on examination that the resemblance is but 
superficial. This has been carefully studied by Cordier, g0, who has 
shown that the cesophageal groove is produced by change in arrange- 
ment of the inner layer of muscle which is contrary to the rule in the 
cesophagus parallel to the direction of the groove. This layer becomes 
enormously developed at the two margins to form the lips of the groove 
and reduced or completely absent in the region corresponding to the 
bottom of the groove. The development and functional completeness 
of the groove is proportional to the degree of specialization of the 
stomach in which it occurs, not the reverse as would be the case if it 
were a persistent primitive structure. Apart from the inadequacy of 
the argument advanced in its support, the hypothesis of cesophageal 
origin is incapable of explaining the facts of structure as we find them. 
One of the strongest arguments against this hypothesis is furnished 
by the stomachs of the monotremes Ornithorhynchus and Echidna which 
as indicated above, are lined throughout by stratified squamous epithe- 
lium and contain no glands unless indeed a portion of the extensive 
group of Brunners glands represent a persistent group of pyloric glands. 
It is only by suppression of the pre-existing glands that such a condi- 
tion could be reached. That the stomach of the Echidna is really a 
stomach and not a portion of the cesophagus is indicated by the obser- 
vation of Oppel, 96, that at a period of development at which the epi- 
thelium of the cesophagus has already become converted into several 
layers, that of the stomach is still a simple cylindrical epithelium. 
Another example of the stomach which while retaining its simple 
external form has undergone extensive internal adaptative modifica- 
tions, is that of Manis javanica in which the stratified squamous epi- 
thelium is found throughout the stomach although the glands are still 
retained in parts. By no process of simple addition from the ceso- 
phagus could such a stomach be produced. 
Equally inadequate is the theory of cesophageal origin, to explain the 
structure of the stomachs of Fiber and certain of the Arvicolide where 
the fundus glands form an isolated group surrounded on all sides by 
stratified epithelium. ; 
It is, of course, at present impossible to definitely exclude the cso- 
phagus from any participation in the formation and development of the 
cardiac sacs. Until more complete histological information is avail- 
able concerning the structure of the stomach in the Cetacea, some reser- 
vation must be made for these forms. But even here it must be remem- 
bered that in constructing his hypothetical series to show the phylogeny 
