394 
MR. W. K. PARKER OK THE STRUCTURE AND DEVELOPMENT 
Behind, these tracts are in contact with the notochord, but in their middle part 
they are narrow, and form the “ posterior basi-cranial fontanel le” of Bathke. 
Then in the part from which the notochord has retired these bands approximate 
up to the oviform pituitary space (py.). 
From thence, forwards, they bear the name of “ trabeculae cranii they are narrowed 
between the maxillary rudiments, and become wider as they meet in front of the great 
opening. 
In front of the pituitary space the trabeculae are close together again, but a dividing 
line of gelatinous tissue can be seen as they pass to the frontal wall ; they will be 
continued as a coalesced process of cartilage into the combined fronto-nasal rudiments ; 
there, of course, they become retral. 
The horizontal sections of the maxillary, mandibular, and hyoid, rudiments are very 
instructive. The first of these (mx.p.) is very massive, for in it the maxillary, palatine, 
and transverse bones will be developed ; but it does not acquire any cartilage in its 
interior. 
But the two first post-oral folds have already acquired a pith, which corresponds 
histologically with the pith of the visceral bars in the first stage of the Salmon 
(“ Salmon’s Skull,” Plate 1, p. 113), it contains a cavity which soon closes (see p. 389). 
The branchial arches do not chondrify ; the auditory sacs are not yet cartilaginous ; 
but both these and the other special sense-capsules soon acquire a coat of thin hyaline 
cartilage. 
Between this stage and the next there is a large difference, but the foundations are 
already laid, and although the building rises almost as rapidly as “an exhalation” 
— for the development and metamorphosis of the cells is only slower than the forma- 
tion of dew — yet from what we have just seen we are enabled easily to interpret what 
we are about to see. 
Third Stage . Embryos of the Snake, from If to 2f inches long . 
The metamorphosis of the head of the Snake’s embryo, whilst the body has not 
doubled its length, is very remarkable. 
The flexure of the head upon itself is' now almost lost (Plate 28, figs. 1-5) ; thus the 
brain segments lie almost in a straight line (fig. 5) ; the mid brain (Ck) forms a very 
projecting, bulbous “ crown.” 
From that part, over the hind brain (C 3 ), the head sinks rapidly, for the cerebellum 
is very small. The skin is now well developed. At present the fore brain does not lie 
flat down upon the floor of the skull, but rises, as in those types that have a septum 
to the orbital region. 
From the primary vesicle (C 1 ) the hemispheres (C 1 ") have budded and developed, and 
from these have budded the olfactory lobes (C 1/ j which now lie in front, supplying 
