752 



ANATOMY OF VERTEBRATES. 



585 



Brain of new-born Kangaroo ; inagn. 

 5 times. lxxV. 



The initial steps in the development of the nervous system of 

 the Mammal closely correspond with those of the Reptile and 

 Bird (vol. II. figs. 39, 135). The brain of the Kangaroo, a fort- 

 night after birth, fig. 585, a, b, has not advanced beyond the 

 condition of that of the embryo chick at the fourth day of in- 

 cubation. Hanging motionless from 

 the teat, like a foetus from the navel- 

 string, its cerebellum, ib. a, c, has 

 not transcended the filmy fold of 

 the cold-blooded saurian type ; but 

 expansion has begun at the base, B, 

 c, of what are destined to become the 

 mammalian lateral lobes. 1 The mesen- 

 cephalon constitutes the main part of 

 the brain : it is a large oblong vesicle, 

 in which the optic lobes, ib. d, begin to be faintly marked off from 

 the ' thalamal ' part, e, overlying the crura cerebri. No organ of 

 the young air-breathing Marsupial offers a greater contrast to 

 that in the new-born placental Mammal than the retarded brain. 

 In form it has got no further than that in the six weeks embryo 

 sheep, but it is firmer in texture : gradually advancing along the 

 Mammalian route, its development stops at a certain point. The 

 superincumbent mass of cerebellum expands, accommodating its 

 ultimate sheet of grey matter to the cranial chamber by transverse 

 folds ; and the lateral lobes stretch out into appendicular lobes, 

 fig. 74, e. The optic lobes, in their growth, show no disposition 

 to special lateral expansion and divergence (as in the bird, vol. II. 

 figs. 42, 44), but swell into a pair of closely united hemispheres : 

 the special mammalian addition is due to growth of neurine in the 

 fore part of the ( valvula vieussenii ' between the ' processus a 

 cerebello ad testes,' which proceeds in Marsupials and all higher 

 Mammals to add a second pair of tubercles (' testes ' of anthro- 

 potomy) to the optic lobes ( f nates ' ib.). Into the cavity of the 

 small hemispheric vesicles, fig. 585, g, i, the ' corpora striata ' 

 first bulge, and are soon followed by the hippocampal protuber- 

 ances : with the former appear the transverse fibres of the anterior 

 commissure, with the latter those of the hippocampal commissure. 

 In Marsupials this is the sole addition to the transverse connec- 

 tions of the hemispheres common to lower Vertebrates : in 

 Placentals, development of the commissural system proceeds to 

 establish the supraventricular mass called ' corpus callosum.' 

 But this is not necessarily accompanied by increased development 

 of the cerebral lobes : the Lissencephala retain the lyencephalous 

 1 lxxV. pi. vii. figs. 11, 12. 



