390 
MR. W. K. PARKER ON THE STRUCTURE AND DEVELOPMENT 
Between the eye and the ear we see the fore part of the hyoid “ root ; ” the root or 
origin of two more folds ; and three visceral clefts. This state of things asks some 
consideration. The two large visceral folds in front of the hyoid rudiment arise from 
a thick mass of mesoblast lying in the valley between the mid and hind brain, below. 
The cleft between these two is the oral cleft (mi), the thick bulbous fold behind is 
the mandibular rudiment (inn.), and the fold in front, which is high above and rounded 
below, is the maxillary rudiment or maxillopalatine fold ( mx.p .). 
The oral cleft is, like those behind the ear, larger above than lower down, and then 
gently widening below. 
These clefts all open freely into the buccal and pharyngeal cavities, and these 
cavities, which meet inside the mandibular rudiments, are lined, the former with 
epiblast and the latter with hypoblast. 
In my last paper (Phil. Trans., 1877, Part II., Plate 21, fig. 4) I showed in the 
unhatched Axolotl’s embryo the future oral cavity filled with mesoblastic cells, which 
cells separated the buccal from the pharyngeal cavity. 
Here, however (Plate 29, fig. 1), we are past that stage, and these two spaces pass 
the one into the other. 
The oral cavity is now almost uniform with the whole space between the head and 
throat, for the maxillary, hyoid, and branchial folds, are all imperfect below, only the 
mandibular have united (fig. 2). What, then, is the relation of the oral to the other 
clefts in further stages ? If we compare this with the next stage, we shall get some 
light upon the subject (Plate 27, figs. 1 and 3 ; Plate 29, figs. 1 and 2). 
In front of the pericardium the mandibular rudiments have united, and thus have 
formed the lower lip (fig. 2, mu.) ; whilst the maxillary rudiments, which seem to be 
narrow as seen from the side, are, indeed, thick masses, directed inwards. (See lower 
view, fig. 2, mx.p.) 
In the next stage (fig. 3) the folds in front of and behind the mouth have become 
widely divergent, so that the apex of the front fold is set on the apex of the hind fold 
at more than a right angle ; hence the gaping oral cavity (figs. 3 and 4, mi). 
The angles of the mouth are the top part of the right and left oral clefts; the 
opening of the mouth is caused by the suppression of the ventral floor of the head ; the 
opening into the cavity of the mouth (buccal cavity) is caused by absorption of the 
mesoblastic mass before spoken of. Hence the mouth of the Yertebrata, like that of 
the higher Invertebrata (all but the lowest), is “ deuterostomatous.” 
In front of the maxillary rudiment we have the foremost cleft ; it is the naso- 
lachrymal (l.cli), and runs between the postero-inferior margin of the eye (at present ; 
afterwards more completely below the eye) and the maxillary rudiment. 
On account of the immense development of the head in front, and the forward posi- 
tion of the nasal sacs, this cleft is a good way behind the foremost visceral fold. 
In the Batrachia (“Frog’s Skull,” Plate 3, figs. 2 and 10) this cleft cannot be seen; 
this arises from the fact that the maxillo-palatine fold is suppressed until the time of 
transformation. 
