NERVOUS SYSTEM OF THE VERTEBRATA. 



421 



mb 



pn. 



and 1 1 8, are representative figures of embryos of various 

 vertebrate forms at a period when the mid-brain forms the 

 termination of the long axis of the body. 



It is generally stated that the 

 cranial flexure is at its maximum 

 at the stage represented in these 

 figures, and there can be no 

 doubt that viewed from the ex- 

 terior the cranial flexure ceases 

 to be so marked a feature, and 

 finally disappears as the embryo 

 gradually grows older ; but 

 though the mid-brain ceases to 

 form the termination of the long 

 axis of the embryo, the flexure 

 of the brain becomes in many 

 forms absolutely more marked ; 

 while in other forms, though 

 stated to diminish, it does not 

 entirely vanish. 



The general nature of the 

 changes which take place will 

 perhaps best be understood by a 

 comparison of figs. 247 and 248 

 representing longitudinal sec- 

 tions at two stages through the brain of an embryo Elasmo- 

 branch. The actual cranial flexure, i.e. flexure of the floor of 

 the brain, is obviously greater in the older of the two brains, 

 though viewed from the exterior the axis of this brain appears 

 to be quite straight. In the younger stage, fig. 247, the mid- 

 brain (mb) forms the end of the long axis of the body, while in 

 the older one the cerebral hemispheres (cer) have grown very 

 greatly, especially forwards and dorsalwards. They have thus 

 come to lie in front of the mid-brain, and to form the end of the 

 long axis of the body, and have at the same time compressed 

 the originally large thalamencephalon against the mid-brain. 

 The same general features may be seen in fig. 250 representing 

 a longitudinal section of the brain of an embryo fowl, and fig. 255 

 representing a longitudinal section of the brain of a Mammal. 



FIG. 248. LONGITUDINAL SECTION 

 THROUGH THE BRAIN OF SCYLLIUM 

 CANICULA AT AN ADVANCED STAGE OF 

 DEVELOPMENT. 



cer. cerebral hemisphere ; pn. pi- 

 neal gland ; op.th. optic thalamus, con- 

 nected with its fellow by a commissure 

 (the middle commissure). In front of 

 it is seen a fold of the roof of the fore- 

 brain, which is connected with the cho- 

 roid plexus of the third ventricle ; op. 

 optic chiasma ; //. pituitary body ; in. 

 infundibulum ; cb. cerebellum ; ati.v. 

 passage leading from the auditory vesi- 

 cle to the exterior ; mel. medulla ob- 

 longata ; c.in. internal carotid artery. 



