OF THE SKULL IN THE COMMON SNAKE. 
387 
very large and projects forward, horizontal in position, in front of the parachordal 
region (Plate 29, fig. 1, m.tr., py., nc.). 
The infundibular (ascending = posterior) part of the fore brain (inf.) has not yet 
united with the small pituitary involution (py.), which is closed above and open below. 
The cranial notochord is slightly curved downwards at its apex ; it has (relatively) 
retreated somewhat from the pituitary space (Plate 29, fig. 1, nc., py.). 
The hypoblastic lining of the alimentary canal has met and united with the epiblast 
of the face and outer part of the rudimentary throat (Plate 27, figs. 1 and 2 ; and 
Plate 29, fig. 1) ; the oral cavity is, therefore, open. 
Behind the mouth those two layers of the blastoderm meet in the clefts between 
the descending visceral folds (see also Foster and Balfour, p. 118, fig. 37); these 
folds are the rudiments of the visceral (haemal) arches and their investments. 
These visceral folds are very rudimentary at present ; behind the mouth there are 
only four marked off from the rest of the tissue at present ; a fifth appears in the next 
stage. 
All but the first two of the post-oral folds are evanescent, and develop no cartilage ; 
in front of the mouth two pairs more of these visceral fringe-like reduplications are 
developed (figs. 1, 2, mx.p., nfp.). 
The structure, the transformations, and the relations, of these descending or hsemal 
growths of the head are of the utmost importance to morphology ; they form the 
palate, the jaws, the hyoid arch, and, in branchiate types, the gill arches. 
They are related to the cranium, the sense-capsules, and the cranial nerves ; 
their interspaces are the mouth, the naso-lachrymal passages, and the post-oral clefts, 
(tympano-eustachian passages, and gill openings). 
So that, above all things, it is necessary to find out how these parts arise at first ; 
from what layers of the blastoderm they are formed ; and with what region, right and 
left, of the body, they may be compared. 
The developing core of these folds, the solidifying floor and walls of the brain-case, 
and the capsules of the principal organs of the special senses — these three groups of 
skeletal structures together make the Vertebrate skull. 
So that it is easy to see that this part of a Vertebrated creature differs as much 
from the rest of the skeleton as the brain and cranial nerves differ from the spinal 
marrow and spinal nerves. Moreover, the capsules and labyrinths of the special sense- 
organs add largely to the sum of differences. 
Both the problem itself, and the methods to be used in attempting to solve it, 
are quite similar to the growth of a flower from axial and foliar rudiments, and the 
patient work necessary for tracing out its steps and stages."' 
The Primordial or Membranous Cranium. — At present, the investment of the brain 
* The vegetative processes that have mysteriously brought the cell-layers of the blastoderm into the 
fundamental “ Sauropsidan” pourtrayed in fig. 1, are surely well worthy to be sought out ; that, however, 
; s the work of the general embryologist, and is, in order, antecedent to my special inquiry. 
