754 PROFESSOR W. C. M‘INTOSH AND MR E. E. PRINCE ON 
fore-brain, to terminate in a pair of large flattened lateral horns, and an internasal plate 
centrally (Pl. XI. fig. 11). These early skeletal structures are the first indications of the 
cartilaginous cranium, but as yet they are formed of closely-aggregated cells, which 
stain deeply, and on account of their density are readily distinguished from the adjacent 
mesoblastie cells out of which they have been differentiated. Whether their cells break 
down or not is difficult to make out, but they undoubtedly become antero-posteriorly 
flattened, and in cross-section the rods under consideration begin to assume a more trans- 
lucent appearance, due to the discoidal character of the constituent cells. Within a week 
or ten days after hatching, these elements are converted into clear boldly-marked nucleated 
cartilage-cells, The large parachordals as they become cartilaginous extend outward, and 
meet to coalesce with the dense cartilaginous floor of the auditory capsules (Pl. VI. 
fic. 9; Pl. XXIII. fig. 2). The trabeculee between the eyes contract, and approach the 
base of the brain in the region near the infundibulum—becoming very narrow as the 
roof of the brain expands. Further forward the trabeculze, however, spread out, forming 
a large anterior plate of cartilage, slightly thinner medially, and more thickened later- 
ally, .e., in the portion forming the cornu (Pl. XI. fig. 11). 
While cartilage thus abundantly develops in the skull, no trace of it is seen in the 
axial skeleton of the trunk—metameric aggregations of mesoblastic cells (PoucHET’s “tissu 
générateur”) alone indicating the points along the notochord where the future vertebrae 
will be formed. During this time also cartilage appears in the form of four small plates 
around each eye, all with a concave surface towards that organ, and formed of large 
cartilage-cells placed over the summit of the eye—beneath and on the anterior and 
posterior surfaces. Whether the first or supraorbital cartilage expands later to form the 
tegmen cranii, and the second to form suborbital elements, while of the remaining two 
one becomes the lachrymal, and the other or postorbital becomes alisphenoid and post- 
frontal, though probable, could not be determined from an embryo in the second or third 
week after hatching. About the middle of the third week, indeed, four series of cartilages 
may be distinguished—(1) the posterior basal, (2) the posterior lateral (auditory), (3) the 
anterior lateral (optic), and (4) the anterior basal. 
The first named constitutes the basis cranii proper (parachordal and occipital elements); 
the second includes a basal auditory plate (Pl. VI. figs. 9, 10), very dense and 
massive, and affording an outer articular surface for the hyomandibular (Am), and 
probably consisting of opisthotic and pro-otic elements, as yet undifferentiated. Above 
the ear a small aggregation of cartilage-cells (epot, Pl. VI. fig. 3) occurs, from which the 
epiotic and supra-occipital are probably formed, while the third series are in a condition 
too early to identify, and are best regarded simply as circumorbital cartilages developed 
at four separate centres on the surface of the sclerotic membrane. The fourth and last 
series occurring at this stage are the trabeculee, with their expanded internasal element 
and the curved lateral cornua. Into the theoretical question of the significance of these 
paired basal bars it is here unnecessary to enter. 
Of the further changes in the skull and facial elements little can be said, as at the end 
