474 Edwin G. Kirk. 
of the whole mammalian series. Second, one may disentangle 
from the generally confused data of the literature, certain con- 
cordant results, which confirm the validity of this assumption 
as to the essential unity of the process throughout mammals. 
Third, and very obviously, a limitation to one form means the 
possibility of more detailed work than is practicable in a compara- 
tive study. The present need seems to be for minute work, like 
that of Toldt on the cat, but utilising valuable, recently developed 
technique, and also one old technique, evidently very often 
neglected nowadays, namely the patient use of serial reconstruc- 
tions in place of the easier reliance on occasional isolated sections. 
Finally, and herein lies the proximal motive in selecting this 
particular species—an abundance of fresh material of every 
stage has been readily procurable. 
I wish to express my sincere gratitude to Dr. R. R. Bensley, 
at whose suggestion this work was undertaken, and under whose 
euidance it has been carried on. Without the aid of certain meth- 
ods devised by him, much of the work on cyto-differentiation 
would have been technically impossible. I am indebted to Miss 
Katharine Hill for the accurate drawings. 
II. HISTORICAL 
Essentially all authors agree that the gastric epithelium is 
derived from the endoderm. Brand ’77, Sewall, ’78 and Kolli- 
ker, ’84 describe a transformation of the original single endodermic 
layer into a stratified epithelium, which later, bysome unexplained 
mechanism, again becomes simple. All recent writers who men- 
tion the subject at all (Toldt ’81, Salivoli 90, Ross ’03) agree 
that the epithelium remains simple from the first, although the 
disposition of nuclei in several planes simulates stratification ; 
of course this does not apply to areas which become stratified 
and remain so, é. g., the left compartment of the field mouse 
stomach (Tépfer. 91), or the pars oesophagea. 
Conéerning the origin of the rudimentary gland tubules, there. 
is great diversity of opinion. The various views may be classi- 
fied under three general groups, depending on whether the tubules 
