572 THE EMBRYOLOGY OF THE FROG 



The cerebral hemispheres appear when the tadpole is about seven 

 millimeters in length, that is, when it is ready to hatch. 



The ventricles are similar to those in the chick. 



As all these thickenings and outgrowths appear, the brain itself 

 seems to straighten the original flexure, but this is only apparent, as the 

 flexure remains, and the infundibulum still extends below and in front 

 of the tip of the notochord. 



The hypophysis (Fig. 333, hy) grows as an inward extension of the 

 surface ectoderm* to meet with the infundibulum. 



THE MID-BRAIN 



There is not much change in this region of the brain except that 

 the ventro-lateral walls thicken, and these thickened portions are known 

 as the crura cerebri. They connect with the wall of the fore-brain. The 

 dorso-lateral walls of the mid-brain form the large, rounded optic lobes. 



The posterior commissure (Fig. 333, cp) forms the anterior limit of 

 the mid-brain. 



The aqueduct of Sylvius (Fig. 303) is the cavity in the mid-brain 

 connecting the third ventricle with the cavity in the rhombenceplralon. 



THE HIND-BRAIN 



There is little, if any, line of demarcation in the frog which divides 

 the hind-brain into metencephalon and myelencephalon. 



The cerebellum (Fig. 333, c) is in the region which is commonly 

 designated as the metencephalon. This organ is quite small in the frog, 

 and appears late in larval life on the dorsal side of the hind-brain. 



The non-nervous thinned-out roof of the fourth ventricle (which 

 covers the dorsal part of the region of the medulla oblongata or myelen- 

 cephalon), (Fig. 333, Ml), forms the choroid plexus of the fourth ven- 

 tricle. 



The floor of the ventro-lateral walls of the hind-brain becomes thick- 

 ened and forms the main nervous pathways to and from the nuclei of 

 origin of most of the cranial nerves. 



The brain gradually tapers into the spinal cord proper at the medulla 

 oblongata. 



The central canal is the cen'tral opening running throughout the 

 length of the spinal cord. It is continuous with the cavities of the brain. 

 This central canal is lined with non-nervous cells known as ependymal 

 cells. The true nerve-cells which go to make up the main portions of 

 the wall of the spinal cord are called germinal cells. These latter are 

 in turn divided into supporting cells or glia cells, and the true functional 

 nerve cells or neuroblasts. 



There is no dorsal fissure in the frog's spinal cord as there is in 

 higher forms, though there is a ventral fissure. 



The gray matter of the cord is formed by the neuroblasts. 



