THE JJHAIN AND SPINAL CORD. 381 



ventral plane (Fig. 158) ; the lateral parts of the floor, and the 

 sides as well, thicken very considerably. The roof of the fourth 

 ventricle, as in the chick, widens out considerably (Fig. 158), 

 and at an early stage becomes exceedingly thin. Immediately 

 behind the cerebellum, the roof of the medulla remains com- 

 paratively smooth, as the velum medullas posterius (Fig. 153, 

 VP) ; but a short way further back it becomes thrown into a 

 complicated series of folds (Fig. 151, XB), which hang down into 

 the fourth ventricle, and between the layers of which the blood- 

 vessels penetrate in. lai'ge numbers to form the choroid plexus 

 of the ventricle. 



The walls of the medulla oblongata consist mainly of longi- 

 tudinal fibres. The pons Varolii (Fig. 153 and 154, PV), the 

 great transverse band of nerve fibres which connects the two 

 halves of the cerebellum together, develops very late ; its posi- 

 tion is indicated, about the eighteenth day, by the sharp rect- 

 angular bend in the floor of the medulla, opposite the cerebellum 

 (Fig. 151). 



Before leaving the brain, it should be noted that, in spite of the 

 complicated foldings which various parts of it undergo, and the 

 extreme thinness to which its walls are reduced in places, notably 

 in the roof of the third, and in that of the fourth ventricle, the 

 cavity of the brain remains a closed one, and its walls are not 

 actually perforated at any place. 



The membranes of the brain, i.e. the pia mater and dura 

 mater, are connective-tissue structures, of mesoblastic origin. 



3. The Spinal Cord. 



The development of the spinal cord of the rabbit need not 

 be described in detail, as in all essential respects it agrees with 

 that of the chick. 



4. The Histological Development of the Brain and the Spinal 



Cord. 



This will be more fully dealt with in the chapter on Human 

 Embryology. 



In the spinal cord, the changes undergone are essentially 

 similar to those in the chick. The original walls of the neural 

 canal give rise mainly to the grey nervous matter, the innermost 



