758 
ME. W. K. PARKER ON THE STRUCTURE AND 
The notochord ( n.c .) is seen to be half the length of the head, or a line and a half, 
and to reach within a very slight distance of the pituitary space ( p.t.s .) : this relative 
but not real retreat of the notochord now bespeaks the first appearance of the post- 
pituitary part of the basisphenoidal region. The notochord is clubbed at its fore end, 
it then is narrower, then thicker between the auditory sacs ( c.l .), and is again somewhat 
diminished as it passes into the atlantal region. 
The broad, thick plate of cartilage (shown as somewhat sliced away, horizontally, 
above the cochlear cavities, C.l., in the figure) which lies right and left of the notochord, 
is the “ investing mass ” (i.v.) of Rathke*. At this stage the “ investing mass ” is rapidly 
passing from simple cartilage into the solid hyaline variety ; it is most dense nearest the 
notochord, and becomes more and more gelatinous as it is traced upwards into the wall 
of the cranium. Hence the commencement of the spinal chord is not as yet bridged 
over by a cartilaginous superoccipital plate, and the cartilage in which the upper part 
of the auditory sacs are imbedded is still in a soft condition. Here it may be at once 
remarked that no separate cartilaginous auditory capsule is formed in the chick (such as 
can be plainly seen in the Tadpole), but the two moieties of the investing mass grow into 
one continuous “ occipito-otic ” cartilage f. At a small distance in front of the cochlear 
cavity (c.l.) the investing mass is suddenly narrowed: this is where the fifth nerve passes 
out ; and this nerve divides the occipito-otic region from the posterior sphenoidal. At 
most, the clubbed anterior end of the notochord is wedged into the very end of the 
basisphenoidal region ; and afterwards it will be seen that the whole of its cranial 
portion becomes the axis of the basioccipital. All the nerves between the fifth cranial 
and the first spinal are related to the investing mass ; so that even if it were ceded that 
the occipital region is essentially vertebral in its nature, yet it must then be looked 
upon, not as one segment, but the non-segmented counterpart of several. On each 
side of the end of the notochord the investing mass at first grows outwards in an angular 
manner (shown in Professor Huxley’s fig. 57 F, and in Plate LXXXI. fig. 2, l.g .) ; these 
angles are the rudiments of the cartilaginous “ lingulae sphenoidales ; ” these parts arise 
from the roots of the trabeculce%. 
* Rathke’s account of this part, in his work on the Snake (Entwickelungsgeschichte der Natter, 1839), is 
thoroughly correct ; I will give it as translated by Professor Htjxiey (Croon. Lect. p. 56, and Elem. Comp. Anat. 
p. 237) : — “ The differences between the basis of the skull and the vertebral column in the earliest embryonic 
condition are : — (1) That, round that part of the notochord which belongs to the head, more of the blastema, 
that is to he applied in the spinal column to the formation of the vertebrae and their different ligaments, is 
aggregated than around the rest of its extent ; and (2) that this mass grows out beyond the notochord to form 
the cranial trabeculae.” 
+ The periotic mass, which becomes perfectly segmented from the occipital sclerotome in many Mammals — 
but in them only in the last or osseous stage of growth — is never cloven in any degree from its surroundings in 
the Fowl ; it is, however, partially cloven off in many Water-birds, by a fenestra which appears between the 
epiotic and the lower edge of the superoccipital. 
X I think that Rathke’s term “middle trabecula” maybe dropped; it appears to me to he merely the 
elatinous precursor of the “ posterior clinoid wall,” and has nothing in common with the basal paired trabeculae. 
— See Huxley’s Croon. Lect. p. 57. 
