THE EVOLUTION OF THE BRAIN IO7 



are arranged to form on the one hand a partial covering or 

 outer skin and on the other a central mass of rather complex 

 organization. The central mass eventually gives rise to 

 almost all of the internal organs of the frog. The outer 

 skin or ectoderm, as it is technically called, sooner or later 

 covers the growing embryo completely and becomes in the 

 end the outer skin of the adult frog. But before this happens 

 other transformations occur. 



As already stated the first changes that lead to the forma- 

 tion of the brain and spinal cord begin about two days after 

 the egg is laid. These changes consist of a thickening of the 

 ectoderm along what will later become the chief axis of the 

 embryo. The band or plate of thickened ectoderm thus 

 formed is called the medullary plate (Fig. 4 a). It extends 

 from what will eventually be the head of the embryo back 

 to its hind end. During the formation of this plate its right 

 and left edges rise and its center is depressed along a line 

 corresponding to the axis of the future animal. In this way 

 a longitudinal groove or ditch is produced which deepens 

 as the plate folds upon itself and sinks into the embryo 

 (Fig. 4 b). As the groove becomes deeper the edges on either 

 side fold over and meet, thus converting the longitudinal 

 groove into a longitudinal tube, the medullary tube, whose 

 walls are the folded plate and whose cavity once com- 

 municated freely with the exterior. Sooner or later this tube 

 becomes entirely closed, breaks away from its mother 

 layer, the ectoderm, and sinks still deeper into the body 

 of the growing embryo (Fig. 4 c). 



The fate of the anterior and the posterior halves of this 

 tube is very different. In the posterior half the walls thicken 

 rather uniformly and give rise to the materials out of which 

 the adult spinal cord is formed. In this process the cavity 

 of the tube diminishes proportionally and becomes the 

 central canal of the adult cord. 



The anterior half of the embryonic medullary tube is 

 destined to become th^ brain. The walls in this part, as in 

 the other, also thicken but the thickenings in the brain 

 region are very local, giving rise to eminences and outgrowths 

 such as the cerebellum, optic lobes, hemispheres, and the 

 like that characterize this part of the central nervous organs. 



