484 
,T. GRAHAM KERR. 
cases, and by a careful and critical use of the two, error can 
be to a great extent eliminated. 
I have, as on other occasions, to acknowledge the valuable 
assistance which I have received from Mr. Maxwell, to whom 
I owe the drawings illustrating this paper, and to Mr. P. 
Jamieson, who has done the necessary section cutting. 
II. Differentiation of the main regions of the Alimentary 
Canal. 
The al imeutary canal of the adult Protopterus or Lepido- 
siren becomes developed out of the mass of primitive endo- 
derm, characterised by its large cells and by the large size of 
the yolk granules with which their protoplasm is laden. The 
almost spherical mass of endoderm of the early embryo 
becomes fashioned into the tubular alimentary canal of the 
adult by a complicated process of what I have somewhat 
loosely termed modelling, the general course of which is 
illustrated by the figures in this paper. It is necessary to 
state quite definitely that, in using the term modelling, I do 
not for a moment mean to suggest that this process is carried 
out by the active agency of the surrounding tissues upon a 
plastic and passive endoderm. On the contrary, I believe it 
to be essential, in all embryological work, to bear constantly 
in mind that the organ is only a part of the organism ; that 
any organ or piece of tissue is throughout its development in 
intimate physiological connection with its surrounding tissues, 
and that to consider it by itself, without reference to these 
surrounding structures, is to make use of a method which is 
almost certain to lead to grave error. The alimentary canal, 
then, of the developing Lepidosiren or Protopterus is, during 
its process of “ modelling,” by no means to be assumed to be 
passive; the whole process is one of co-operative activity 
between the endodermal rudiment and the mesodermal struc- 
tures in relation with it. 
The first stage of the modelling of the alimentary canal is 
that in which a narrow anterior part — the fore-gut — becomes 
