304 
HENRY ORR. 
and the hypophysis and notochord change their positions with 
relation to each other. Fig. 13 e shows the anterior end of 
the notochord, which in this embryo is at some distance from 
the hypophysis, while the floor of the hind-brain ( H . B.) is 
widely separated from the infundibulum. There is a median 
thickening of the hypoblast extending from the anterior end 
of the notochord down to the hypophysis. This thickening 
seems to disappear very quickly after formation. It seems 
possible that this median thickening may be homologous with 
that foremost part of the notochord which in the Lizard and 
in the Mole extends as far as the epiblast at the hypophysis. 
In the Anura at an early stage there is a layer of mesoblast 
extending across the median line between the anterior end of 
the notochord and the hypophysis-rudiment. Why the meso- 
blastic product of the hypoblast along the median line at this 
region does not become differentiated into notochord in the 
Amphibia, as it does in the Lizard and the Mole, may be ex- 
plained perhaps by the changes which immediately succeed 
this stage — changes which would be hindered by a developed 
notochord in this region. The changes thus referred to are 
exhibited in fig. 17 g. Here the secondary cranial flexure has 
appeared in the hind-brain, and the floor of the hind- brain is 
pushed against the infundibulum, causing the latter to be 
slightly compressed. At the same time the bending floor of 
the hind-brain has pushed the notochord downward, so that 
the anterior end of the developed notochord touches the hypo- 
physis. (These changes of position are of course to be under- 
stood only in terms of relative topography as the absolute 
changes of location cannot be ascertained. Thus, the changes 
might be accounted for by supposing the secondary cranial 
flexure to lift the anterior part of the brain and head upward ; 
but the former view lends itself more readily to the explana- 
tion of the facts, and admits of more extended homologies). 
As nearly as can be judged from the more limited number 
of my specimens of Triton, the method of development during 
the above-described stages is exactly the same in Triton as in 
Amblystoma; though I should add that my youngest stage of 
