DEVELOPMENT OF MAMMALIAN SKELETON. 753 



superficies and proportions of the superincumbent masses of the 

 prosencephalon. In the Gyrencephala these extend backward 

 over the mesencephalon, and more or less of the cerebellum : from 

 the lissencephalous condition transitorily shown by the human 

 foetus, fig. 125, the middle lobes, d, progressively grow into 

 posterior ones, finally extending in Archencephala above and 

 beyond the cerebellum, and acquiring the proportions and condi- 

 tions of the posterior horns of the lateral ventricles and ( hippo- 

 campi minores' peculiar to and characteristic of the human 

 brain. 



§ 408. Development of Mammalian Skeleton. — The notochord 

 early begins to show a series of dilatations answering to the 

 later intervertebral spaces. 1 In the embryo head the blastemal 

 coverings of the piers of the anterior cephalic haemal arch (maxil- 

 laries) project freely, and appear as processes of the second (man- 

 dibular) arch : only the proximal parts of the third (hyoidean) 

 arch are indicated by indentations, and the piers do not project 

 freely. The chief developmental mammalian modification arises 

 from the proximity of the precociously and rapidly growing ap- 

 pendages of the acoustic sense-organ ('ossicula auditus ') peculiar 

 to the class : accompanied with a reduction of the proximal part 

 of the mandibular arch to the support of the tympanum, and 

 with a slight forward dislocation of the distal part of the arch. 

 In Monotremes the tympanic (vol. ii. fig. 197, 28), large and 

 well-ossified in the blind and naked young, has its growth ar- 

 rested and diverted by the rapid and excessive growth of the 

 malleus, which becomes anchylosed to the tympanic by its long 

 process, o, whilst its ( manubrium,' c, gives attachment to the 

 radiating fibres of the muscle of the ear-drum. The incus, b, is 

 represented by a small and early confluent epiphysis. The colu- 

 melliform stapes d is relatively small as in other Mammals. The 

 base of the mandible extends inwardly to join the tympanic, and 

 its articular surface is also extended outward, as in the Bird : the 

 conformity with the Chick in the relations of both tympanic and 

 mandible to the primary and transitory cartilaginous haemal arch, 

 and the plain homology of the ossicle, b, with the better developed 

 incus of higher Mammals, are decisive against the revival of 

 Reichert's ill-founded conclusion as to the homology of the Mam- 

 malian incus with the os quadratum (tympanic) of Birds and 

 Reptiles. In the mammary Kangaroo the tympanic, embracing 

 by an upper bifurcation the hind part of ( Meckel's cartilage,' 

 develops a convexity below adapted to the inner side of the 



1 CCCXXIIl". 



VOL. III. 3 C 



