382 THE HABB1T. 



layer of cells forming the epithelial lining of the central canal. 

 The white matter develops later ; in the spinal cord it appears 

 on the eleventh day, the ventral and lateral bands of white 

 matter being formed practically simultaneously. The central 

 canal of the spinal cord remains a narrow vertical cleft until 

 about the sixteenth day, when its dorsal part becomes obliterated, 

 as in the chick, preparatory to the formation of the dorsal 

 fissure. 



In the medulla oblongata, the arrangement of white and 

 grey matter is essentially the same as in the spinal cord, the 

 white matter forming, about the eleventh day, on the outer surface 

 of the grey matter ; in the later stages the relations between 

 white and grey matter become much more complicated. 



In the fore-brain the conditions are somewhat different. 

 The walls of the hemispheres, which at first are very thin, 

 become early differentiated into an outer layer of rounded 

 elements, which later on give rise to grey matter, and an inner 

 epithelial layer, which becomes the epithelial lining of the 

 ventricle. About the sixteenth or seventeenth day, bands of 

 white fibres grow upwards from the crura cerebri, through the 

 optic thalami and corpora striata, and make their way between 

 the two layers of the wall of the hemisphere ; while a little 

 later a very thin superficial layer of white matter forms on the 

 surface of the brain, outside the grey matter. In this way 

 the characteristic distribution of white and grey matter in the 

 hemispheres of the adult is brought about. 



5. The Peripheral Nervous System. 



The general history of the peripheral nervous system is the 

 same in the rabbit as in the chick ; but the earliest stages of 

 development have not yet been worked out in such detail. By 

 the ninth day both spinal and cranial nerves are established, 

 and by the eleventh day (Fig. 165) all the principal branches of 

 distribution are present. 



The Cranial Nerves. 



I. The olfactory or first cranial nerve. The time of first 

 appearance of the olfactory nerve in the rabbit has not been 

 definitely determined. The nerve is, however, clearly recog- 

 nisable, as a short stem, connecting the cerebral hemisphere 

 with the olfactory pit, before the olfactory lobe is formed. 



