126 THE FEOG. 



end of the fore-brain begins to grow forwards as a median thin- 

 walled vesicle of the hemispheres ; this steadily increases in size, 

 but, up to the time of the formation of the mouth, remains un- 

 divided. At this stage (Fig. 64, BC), it is approximately 

 spherical, and about equal in size to the mid-brain : its roof and 

 anterior wall are both extremely thin, but its side walls are much 

 thickened, so that the central cavity is compressed laterally. 



The vesicle continues to increase in size, but remains single 

 and undivided up to a stage slightly later than that shown in 

 Fig. 65, when a division between the two hemispheres appears. 

 This is effected by the roof and anterior wall becoming folded ver- 

 tically along the median plane ; the fold, which is continuous pos- 

 teriorly with the choroid plexus of the third ventricle, projects into 

 the cavity of the vesicle, and partially divides this into right and 

 left halves, which become the lateral ventricles of the hemispheres. 



By further growth forwards of the hemispheres, with thick- 

 ening of their walls, the proportions of the adult brain are 

 gradually acquitted ; the brain at the time of the metamorphosis 

 being practically identical with that of the fully formed frog. 

 The anterior or distal extremities of the hemispheres become the 

 olfactory lobes ; these are at first separate from each other, but 

 ultimately become fused together along their inner surfaces. 



3. The Development of the Peripheral Nervous System. 



It will be convenient to give first a general description of the 

 early stages of development of the peripheral nervous system of 

 the frog, and then to deal separately with the cranial and the 

 spinal nerves in regard to the later phases of their formation. 

 There are still many points on which our knowledge of the 

 development of the nervous system in the frog is imperfect 

 and unsatisfactory. 



The early stages of development of the nerves. The dorsal 

 roots of the spinal nerves, and the majority of the cranial nerves, 

 arise in closely similar manner, and at a very early period. The 

 first commencements are seen in embryos which are still almost 

 spherical, and in which the neural plate and neural ridges are 

 just commencing to form, but have not yet begun to fold in to 

 inclose the neural tube (Fig. 57). 



The neural plate is formed, as described above, by thickening 



