THE EARLY DEVELOPMENT OF THE FROG 121 



At the opposite end of the body the enlarged brain protrudes, 

 forming a head region faintly indicated externally. A sagittal 

 section (Fig. 37, A) shows that the future regions of the brain 

 are but faintly marked out. The chief characteristic of the 

 brain is its abrupt bending or flexure around the anterior tip 

 of the notochord; the region immediately in front of the chorda 

 is that of the future mid-brain, while the large fore-brain lies 

 entirely below the level of the chorda and the remainder of the 

 neural tube. In the mid-line, just beneath the end of the fore- 

 brain, a tongue-like proliferation of ectoderm cells extends 

 inward a short distance. This is the rudiment of the hypophysis. 



The simple rudiments of the chief sense organs are also indi- 

 cated at this early stage. The eyes are distinguishable, even be- 

 fore the brain closes, as small patches of deeply pigmented ecto- 

 dermal epithelium in the antero-lateral regions of the medullary 

 plate. When the neural tube is completed, they form a pair of 

 hollow ventro-lateral outgrowths from the fore-brain to the 

 superficial ectoderm. Frequently they can be seen externally, 

 marked by a pair of slight darkened elevations, either side of the 

 fore-brain region (Fig. 22, E, F). The ears are indicated as a 

 pair of thickened patches of the inner or nervous layer of the 

 ectoderm opposite the hind-brain region. They are scarcely 

 visible externally at this time. The olfactory organs develop as 

 thickened circular patches of ectoderm below and in front of the 

 optic rudiments. At this stage a pair of slight depressions may 

 sometimes be detected on the surface, marking the future 

 olfactory pits. 



2. The Notochord 



By the time the neural tube is completed the chorda has 

 become completely delaminated from the outer surface of the 

 endoderm, except only in the region of the primitive streak 

 where its formation is still progressing posteriorly. The separa- 

 tion of the chorda from the endoderm, or rather the enteroderm 

 (see below), occurs in the posterior direction, beginning near 

 but not quite at the anterior tip. 



