NERVOUS SYSTEM. 235 



rounded mass, more easily accommodated and protected, as 

 we see in birds and mammalia. The spino-cerebral axis fills 

 the osseous cavities which protect it, and the form and size 

 of the upper part of the head now correspond with those of 

 the enclosed brain at every period of life. The spinal chord, 

 become more rounded in form, has an encreased proportion 

 of grey ganglionic matter in its interior, and the white 

 fibrous medullary portion is encreased in the brain. The 

 enlarged cerebral parts being now collected into a more 

 compact form, the hemispheres of the brain extend back- 

 wards to cover the optic lobes, and come into contact with 

 the greatly encreased cerebellum. The olfactory tubercles 

 have much diminished, but retain their tubular communica- 

 tion with the still small ventricles of the cerebral hemi- 

 spheres. The cerebral hemispheres still destitute of convo- 

 lutions and partially bilobate, and the median lobe of the ce- 

 rebellum deeply laminated and sulcated transversely, are the 

 parts here most developed in the cranium, and the white 

 fibres of the great cerebral commissure, the corpus callosum, 

 have begun to shoot across and unite the hemispheres. Not- 

 withstanding the great transverse extension of the cerebral 

 mass in adult birds, and its sudden tapering forwards to the 

 olfactory nerves, giving the whole brain a triangular appear- 

 ance, it presents the same lineal arrangement and longitudi- 

 nal extension of its parts in the embryo, which are preserved 

 as the permanent character of the nervous axis of fishes. 

 This is easiest observed by examining its development in the 

 transparent area of the cicatricula in the egg of the fowl, 

 where we perceive after two days 5 incubation (Fig. 97. E,) the 

 two halves of the spinal chord (E. a,) united posteriorly, and 

 already forming the vesicular enlargement (97- E. ,) corres- 

 ponding to the caudaequina and pelvic dilatation, orrhomboidal 

 sinus of the adult. The cervical and dorsal vertebrae begin to 

 embrace the anterior portion (97. E./,) of the chord, and 

 three vesicular enlargements ar,e seen on the cephalic portion 

 of the nervous axis. The posterior (97- E. c,) of these en- 

 largements forms the rudimentary lobes of the medulla ob- 

 longata, the middle dilatation (97. E. d,) constitutes the out- 

 line of the optic lobes, the anterior (97- E. e,) and smallest 

 cephalic enlargement forms the embryo condition of the 

 cerebral hemispheres in the chick, and all these lobes are 



