160 DEVELOPMENT OF THE FROG'S EGG [Cn. XV 



the tube, forming a broad sheet of cells on each side, continu- 

 ous with the dorsal edges of the closing tube. These lateral 

 sheets are very large and conspicuous at the anterior end of 

 the nerve-tube. The subsequent history of these structures 

 will be followed later. 



The first part of the medullary tube to close, is the antero- 

 median portion, and from this point the closure of the tube 

 extends anteriorly and posteriorly. At the anterior end, the 

 tube remains open latest ; at the posterior end, the medullary 

 folds arch over the blastopore, as already described. 



When the medullary folds have met along the mid-dorsal 

 line, the apposed edges fuse, and the outer layer of ectoderm 

 then becomes continuous over the outer surface of the embryo. 

 A part of the same layer has been cut off and lines the cavity 

 of the neural tube. The nerve-tube soon loses all connection 

 with the overlying ectoderm (Fig. 40). 



The anterior end of the nerve tube is larger than the rest, 

 and this end is at first bent down nearly at right angles to the 

 long axis of the more posterior portion (Fig. 37, A). The 

 bending begins at the front end of the notochord. A slight 

 transverse infolding of the wall of the anterior end of the tube 

 takes place soon after its closure, and later another transverse 

 infolding occurs, still further forward. As a result three divi- 

 sions or vesicles of this region are produced. They correspond 

 to the fore-brain, mid-brain, and hind-brain respectively. The 

 fore-brain (Fig. 37, FB) is the large anterior vesicle. From it 

 develops later the third ventricle, the pineal body, the infun- 

 dibulum, the optic vesicles, and the cerebral hemispheres. The 

 mid-brain (Fig. 37, B) is the smallest of the three divisions, and 

 gives rise to the optic lobes and to the Sylvian aqueduct. The 

 hind-brain is continued into the more posterior medullary tube. 

 It lies in the same plane with the medullary tube, and repre- 

 sents only a somewhat enlarged part of the tube. The hind- 

 brain becomes the medulla oblongata, and from its roof the 

 cerebellum is formed. 



The roof of the fore-brain is very thin. Near the middle of 

 its upper margin an evagination is formed, which is, at first, 

 only a hollow diverticulum (Fig. 37, B), but when the tadpole 

 leaves its capsule, the peripheral end of the outgrowth forms a 



