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 (PI. 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 

 mesoblastic 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 (PI. VI. 

 fig. 9 ; PI. XXIII. fig. 2). The trabeculse 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 trabecules, however, spread out, forming 

 a large anterior plate of cartilage, slightly thinner medially, and more thickened later- 

 ally, i.e., in the portion forming the cornu (PI. XL 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 

 generateur") 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 (PI. VI. figs. 9, 10), very dense and 

 massive, and affording an outer articular surface for the hyomandibular (hm), and 

 probably consisting of opisthotic and pro-otic elements, as yet undifferentiated. Above 

 the ear a small aggregation of cartilage-cells (epot, PI. 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 trabeculae, 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 



