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E. W. MACBRIDE. 
undergoing mitosis shows the same rounded form, which, 
when occurring at tlie dorsal lip of the blastopore, Cerfontaine 
regarded as an infallible proof of ectodermal origin. 
In figs. 7 a aud 7 h (PI. 2) a median and a lateral section 
of a still older embryo are represented. The nerve-plate is 
now unmistakably present, as the flattened dorsal surface of 
the embryo shows. The abrupt passage from ectodermal to 
endodermal nuclei at the point x is also evident. Now, how- 
ever, we may see the extent of the histological difference 
between the cells forming the roof and those forming the 
floor of the archenteron, on which Lwoff and Cerfontaine lay 
so much stress, and which has also been referred to Morgan 
aud Hazen. The yolk-granules in the cells of the roof {ch.) 
are becoming fewer, aud there are mixed with the large ones 
smaller granules, and they do not stain quite as deeply as 
those in the cells forming’ the apex or anterior end of the 
archenteron. All these appearances are due to one cause ; the 
yolk-granules are being more rapidly used up in the cells 
forming the roof of the archenteron than elsewhere, and this 
in turn can be referred to two causes : first, these cells have 
undergone more rapid multiplication than those forming the 
rest of the archenteron; and secondly, it is from these cells 
that the notochord will eventually be formed, aud the 
incipient disappearance of the yolk-granules may be in part an 
anticipation of that vacuolisation which distinguishes the 
notochordal cells from those forming tlie I’est of the archen- 
teron. It is in any case much less observable in the more 
lateral section represented in fig. 7 h. 
We have now traced the development of the gastrula step 
by step from the stage of the flattened blastula up to what we 
may term the thimble stage, in which the nerve-plate is 
clearly developed and about the orientation of which there 
can be no possible doubt, and I consider that this is the best 
answer to the charge of Morgan and Hazen that I confounded 
the two lips of the blastopore. No such close series of stages 
is represented in their paper. The gastrula, whilst still 
retaining its wide-open blastopore, has become deeper and 
