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HENRY ORR. 
accompanied by a corresponding change in the position of the 
hypophysis rudiment ( Hph .), so that the latter continues in the 
same topographical relation to the anterior fold. The rudiment 
of the hypophysis extends a short distance posterior to the 
limit of the anterior fold (o. g.). 
The next three stages (figs. 21, 22, 23) illustrate the further 
development of these parts. The most striking changes are 
the increase of the cranial flexure and the growth of the dorsal 
part of the fore-brain. (This latter is not so well shown in 
fig. 21 owing to the obliquity of that section.) It is evident 
that these changes would cause a relative change of position 
of the point marked o.g. in fig. 20. In two of the sections 
(figs. 22, 23) may be seen a slight groove (o.g.) in the mor- 
phologically anterior surface of the brain. This groove lies at 
first between the optic stalks, and ultimately just anterior (or 
morphologically dorsal) to the chiasma. I have not been able 
to absolutely demonstrate that the groove (o. g.) is developed 
from the point o.g. in fig. 20, but the evidence in favour of the 
view that such is the case seems to me so strong that I have 
been forced for the present to accept that conclusion. In the 
Lizard the primitive opening of the brain extends down the 
anterior surface of the brain to a point between the optic stalks, 
and in the Lizard there is also a similar groove at that point. 
In the present case we have only to imagine that owing to the 
increase of the cranial flexure and the growth of the fore-brain 
the point o.g., fig. 20, has receded relatively in a posterior 
direction, until it reached the point o. g., fig. 23. In figs. 22 
and 23 such a relative posterior recession of the groove o. g. is 
perfectly evident. This relative recession is due chiefly to the 
greater growth in the region in front of the point o. g. It will 
be seen that in all these five embryos (19 — 23) the posterior 
end of the hypophysis-rudiment lies at about the same distance 
behind the region of the point o.g., but the lower jaw advances 
continually until it extends anteriorly beyond the posterior end 
of the hypophysis and beyond the point o. g. This process of 
growth is essentially the same in the Frog as iu Amblystoma 
and Triton ; but in the Frog the growth of the dorsal part of 
