372 THE BABBIT. 



The general history of the development of the nervous system, 

 both central and peripheral, of the rabbit is closely similar to 

 that of the chick, and it will be only necessary to describe in the 

 present section the points of more special interest, and in parti- 

 cular those characteristic of Mammals as contrasted with Birds. 



2. The Brain. 



Cranial flexure commences towards the end of the ninth day 

 (Fig. 146), before closure of the neural canal is completed ; and 

 proceeds rapidly. By the tenth day (Fig. 1 17) the brain and 

 spinal cord are closed along their whole length; cranial flexure 

 is strongly pronounced ; and the head of the embryo, mainly 

 owing to the rapid growth of the brain, has acquired a shape, and 

 proportions, similar to those of a chick embryo towards the close 

 of the fourth day of incubation. 



By the twelfth day (Fig. 161) the head has increased con- 

 siderably in size, and, when the embryo is viewed from the side, 

 appears to be bent twice at right angles ; the first bend being near 

 the junction of the brain and spinal cord, opposite the reference 

 line HY in Fig. 161; and the second bend being marked by the 

 mid-brain. BM, which forms the most prominent part of the brain 

 at this stage. 



Sagittal sections of twelve-day embryos (Fig. 150) show 

 that flexure has really taken place to a far greater extent than 

 is apparent from the surface. Following the floor of the brain 

 from behind forwards, there is first, at the junction of spinal 

 cord and mid-brain, between the reference lines TH and CH in 

 Fig. 150, a rather sharp bend, ventralwards, of nearly 90 ; this is 

 corrected a little further forwards by a second and more gradual 

 bend dorsalwards, at the level of the reference line PT. At the 

 base of the mid-brain, BM, there is a very sharp and sudden 

 bend of about 180, by which the infundibulum, IN, and the 

 floor of the hind-brain are brought almost into contact with each 

 other. In front of the infundibulum the floor of the brain 

 again bends dorsalwards, and nearly at right angles. 



These flexures are even more strongly marked in the later 

 stages of development, the angles formed by them becoming 

 sharper and more pronounced. This is well shown in Fig. 151, 

 which represents the condition of the brain 011 the eighteenth 

 day, as seen in sagittal section. The extreme sharpness of the 



