OF THE SKULL IN THE UKODELOUS AMPHIBIA. 
535 
bent condition of the head, lie below the apex of the notochord, behind the pituitary 
floor, and above the cleaving lips*. 
The notochord (Plate 21. fig. 4, nc.) reaches the pituitary body by its sheath ; it is 
turned downward, and this deflection takes place at both ends (see Gotte’s and Balfour’s 
plates). At this stage all obscurity as to the anterior termination of the notochord 
has vanished, and the solid chromic-acid preparation, whose section is here figured, gave 
precisely the same results as similar sections of early embryos of the Salmon (see Phil. 
T rans . 1873, plate iii. figs. 3 & 5, nc.). In both these types the watery tissue of the 
notochord ceases close behind the pituitary body ; but its dense sheath passes forwards 
and upwards beyond the hinder margin of the pituitary body and infundibulum. This 
Axolotl embryo is at an earlier stage than those of the Salmon, and its notochordal 
sheath wedges in between the hind and mid brain. 
This empty sheath is suddenly folded, as it were, upon itself, and the lower or 
returning layer develops a cup-shaped process at right angles with the notochord, which 
forms a well-fitted “rest” for the globular pituitary body {py.). This scooped plate, in 
turn, rests against a loose mass of mesoblastic cells, which are evidently parental to 
the stroma which afterwards ossifies to form the parasphenoid. 
Thus, notwithstanding that the cephalic end of the notochord is, like the caudal end, 
turned downwards, it seems to seek to grow through the neural region to the upper 
wall of the head nearly parallel with an ideal line passing through the pituitary and 
pineal rudiments [pn.,py.). 
The apex of the notochord first straightens, and then acquires more or less of an 
upward curve. I see this in the embryo of Scyllium (11 lines in length), and it is shown 
in my figured sections (the third, fourth, and fifth) of the embryo Salmon’s head. But 
this is shown very remarkably in the hot-blooded types, as the Fowl and the Pig (“ Fowl’s 
Skull,” plate lxxxi. fig. 3, nc. ; and “ Pig’s Skull,” plate xxviii. fig. 6, nc.). 
I am careful to notice the direction taken by this axial and most fundamental part 
of the skeleton ; around its cranial end cluster most of the difficulties that beset the 
labourer in this field f. 
* These cells evidently correspond to those described by Mr. Balfour as obscuring the view of the notochord 
in his stage “6” of the Selachian embryo ; and as I must now describe the notochord of the Axolotl, it will 
be profitable to give his observations on that of the Shark. “ Another structure which became developed in 
even a younger embryo than ‘ C ’ is now for the first time visible in the living embryo. This is the notochord : 
it extends from almost the extreme posterior end of the embryo. It lies between the ventral wall of the spinal 
canal and the dorsal wall of the intestine ; and round its posterior end these two walls become continuous with 
each other (plate 24, G). Anteriorly the termination of the notochord cannot be seen ; it can only be traced 
into a mass of mesoblast at the base of the brain, which there separates the epiblast from the hypoblast.” 
This cell-mass is seen coloured rusty red in the figure of the embryo of Bombinator igneus (‘ Early Stages in 
the Development of Vertebrates,’ plate 1. fig. C 3), apid is black in the woodcut in the paper on the Elasmo- 
branchs (p. 545, figs. B, C). With these papers and descriptions before him the reader will much better be 
able to follow my description and to understand the figure (4). 
f The ventral ends of the non-chondrified visceral bars are coloured (fig. 4) to assist the eye ; they are 
becoming quite distinct and solid : below the hinder of these, and in front of the unused yolk-cells, the rudimen- 
tary heart (h) is seen ; it is partly subdivided by a constriction. 
