536 
MR. W. K. PARKER ON THE STRUCTURE AND DEVELOPMENT 
The latest observations that I have seen on the origin of the notochord are those made 
by Mr. Balfour (Dev. of Elasm., Journ. of Anat. and Phys. vol. x. p. 682); these 
quite corroborate his former assertions, namely, that it is developed from the “ hypo- 
blast.” I must refer the reader to his paper for the proofs of this, and for the views of 
other anatomists, and proceed to describe the morphology of the Axolotls skull in the 
soecnd stage. 
Skull of Axolotl. — Second Stage. Embryos unhatched, and measuring from 
4 to 4^ lines in length. 
In embryos one fourth longer than the last (Plate 21. figs. 5-7) a great change 
has taken place ; the mesocephalic flexure is fast vanishing, and the mouth, now cleft 
through, is nearer the fore part of the head. 
The hind brain (C 3 ) is now more perfectly roofed in, and is relatively much smaller ; 
the mid brain (C 2 ) projects but little over the fore brain (C 1 ). 
The mid brain is now symmetrical on its upper surface, being equally divided into two 
crescentic elevations, placed back to back ; these developments in the roof of the mid 
brain are the optic lobes*. 
The azygous rudiment of the cerebral hemispheres now lying near the fore front of 
the head, the part attached to the hinder margin of the investment of this vesicle is of 
necessity brought near to the front : this part is the naso-frontal process ( f-n.j) .), and 
is the rudiment of the upper lip and of the parts within and behind it. 
On each side of this naso-frontal selvedge the olfactory rims ( ol .) are more marked 
out, but they still retain their place below the head as in Selachians. 
The eyeballs (e) are almost formed, a fissure existing where there was an open “ hilus ” 
before ; the distinctly globular ear-sacs ( au .) still show the primitive opening or invo- 
lution. 
The fore part of the lower lip, and its contained mandibular rudiments, is projected 
forwards, and being broad makes the mouth to be a transverse cleft, as in the Selachians : 
but there is a considerable lateral extension of the opening ; this is overhung by large 
labial growths from the so-called “maxillary rudiments;” they resemble the “flews” 
of a hound. 
The elevations caused by the enclosed mandibular and hyoid bars, which did stoop 
forward above, now lean backward ; this direction increases more and more, and then 
* Balfour, ‘ Development of Elasmobranch Fishes,’ p. 563. At p. 560 the round fore brain is spoken of as the 
“ impaired rudiment of the cerebral hemispheres ; ” from these the “ optic vesicles,” the essential part of the 
eyeballs, arise. In a note to p. 560, Mr. Balfour gives another account of the origin of the “ thalamen- 
cephalon ” to that in Prof. Huxley’s 4 Elements,’ where (p. 131) this part, “ the vesicle of the third ventricle,” 
is said to arise from the posterior part of the fore brain. The note runs as follows : — “ The part of the brain 
which I have here called mid brain, and which unquestionably corresponds to the part called mid brain in the 
embryos of higher Vertebrates, becomes in the adult what Miklucho-Maclay and Gegenbaur called the vesicle 
of the third ventricle or thalamencephalon. I shall always speak of it as the mid brain.” 
