GASTRULATION IN BIRDS. 
151 
Pi-om these experiments it is perfectly plain that the germ 
I'ing representing the lips of a posteriorly placed blastopore 
provides the material for growth in length thus — the mid- 
dorsal part for the mid-dorsal region of the embryo, the 
nerve-cord and notochord, the lateral parts for the sides, the 
venti'al part for the ventral surfaces. I ma}' refer the reader 
to some remarks on this in my paper on Teleostean devclop- 
Tiient, ‘Guy’s Hospital Reports,’ vol. Ixi, 1907. 
If, therefore, Patterson’s account of the formation of the main 
axis of the pigeon by concrescence is correct it is interesting 
and remarkable, but at any rate it is not like the Teleostean. 
Again, where in the animals most closely connected with 
the birds in adult characters, the reptiles and mammals, can 
we possibly find the slightest hint of any process either of an 
involution of a fi’ee edge or a process of concrescence ? 
If we turn from such general considerations to his actual 
experiments we are not coTivinced by them. 
In the first place there is some, but not much, chance of 
mistake in the orientation. Patterson says that in the 
pigeon’s egg the embryo lies with its longitudinal axis at an 
angle of 45° with the longitudinal axis of the egg (“ chalazal 
axis ”) in 90 per cent, of eggs. Presumably he discarded 
experiments in which on the development of the embryo it 
was found to deviate from 45°. 
Exp. I. (Operation 33 a liours, examination 37 hours after 
the estimated time of fertilisation.) 
The posterior margin of the blastoderm, at this time a free 
edge, was injured by cauterisation before it had become 
involuted, which injury “ought to be carried down beneath 
the blastoderm during the course of further development, that 
is, it ought to be found in the entoderm ” (p. 88). 
’File truth of this contention is supposed to be demonstrated 
by a photograph (fig. 66). There is nothing to indicate 
which is anterior or posterior end, but I take it that the 
number “66” is close to where the edge of the blastodenn 
should be, and that the space under the lettei’s “op” repre- 
sents the deficiency in the entoderm. We are asked to compare 
