DEVELOPMENT OF THE SKULL IN THE PIG. 



297 



vertebral structure ; and the whole basisphenoidal territory, small as it is in the Pig, is 

 very compound, having its origin in the two apices of the investing mass, in the apices 

 and posterior end of the commissure of the trabecular rods, in a chondrified part of the 

 cranial wall (related to the investing mass merely by coalescence at its postero-inferior 

 angle), and, lastly, in a secondary growth of cartilage beneath the pituitary body. This 

 last growth of cartilage is found in Sharks and Batrachians, but not in Teleostean 

 Fishes nor in Lizards and Birds. 



The basisphenoidal territory may be understood by drawing one imaginary line across 

 the end of the trabecular commissure (Plate XXIX. fig. 4, tr.cm.), and a second across 

 the apices of the investing mass, leaving a short tract in front of the posterior line ; the 

 figure itself ends at this second line. 



The cranial wall in the anterior sphenoidal region is altogether soft and gelatinous 

 at present (Plate XXIX. fig. 5, a section through the primary cerebral vesicle, thala- 

 mencephalon, and its two outgrowths, the hemispheres) ; beneath this part the trabe- 

 cular bands are now fairly differentiated (fr.), and these form the lower half of the 

 compound presphenoid, as we shall soon see. These are the three proper cranial regions, 

 corresponding to the cerebral vesicles, but not in any proper morphological sense 

 answering to the divisions of the body, the somatomes ; no other segment can be found, 

 for the immense outgrowths of the first cerebral vesicle lie upon the nasal roofs. 



Even the posterior sphenoid loses what little relation it had to the notochord, which 

 is absorbed by the basioccipital, and is largely formed by borrowing substance from the 

 first facial arch ; but the anterior sphenoid is a mere chondrification of the middle layer 

 of the membranous cranium, the two wings mutually sending downwards an azygous 

 plate which coalesces with the common crest of the trabecular bars. 



The Sense-capsules. — The extent of the olfactory region is at present very small; 

 afterwards the whole labyrinth takes up three fourths of the cranio-facial length. The 

 squarish septum (Plate XXYIII. fig. 6, s.n.) looks almost directly downwards beneath 

 the first cerebral vesicle, and the double roof of the labyrinth has the relation of an 

 eave to the cerebral roof. The most projecting kidney-shaped part of the nasal roof 

 is the "ala nasi;" and both lateral and front views of the embryonic head show how 

 this is, as it were, grafted on to the upper surface of the down-bent trabecular bars. A 

 great difficulty is got rid of here, which has cost me much trouble; for the alae nasi 

 are not formed out of the substance of the trabeculse, nor can they be considered, in 

 the adult, merely the front and portico of the roof of the nasal labyrinth ; they com- 

 pletely coalesce with the trabecular knuckles ; and the rooting, ossified snout of the adult 

 Pig is of a compound nature, principally, however, formed of the genuflection of the 

 first visceral arch. 



The passage from the outer to the inner nostril (Plate XXVIII. fig. 4, e.n., i.n.) is 

 already tortuous (Plate XXVIII. fig. 7, a vertical section beyond the septum nasi); 

 for already the mucous membrane has been thrown into baggy folds, into which soft 

 outgrowths of the roof are entering, afterwards to become a labyrinth of cartilaginous 



MDCCCLXXIV. 2 K 



