THE LATER DEVELOPMENT OF THE FROG .129 



boring regions. Just opposite the tip of the chorda the floor 

 of the brain becomes slightly thickened as the tuberculum poste- 

 rius, and in the roof, obliquely upward and forward from this., 

 appears a rather extensive dorsal thickening (Fig. 41). With 

 the aid of these landmarks we may map out the location of the 

 future brain regions. A plane passing from the tuberculum 

 posterius in front of the dorsal thickening, marks approximately 

 the limit between the primary fore-brain or prosencephalon, and 

 the mid-brain or mesencephalon; while a plane passing from the 

 tuberculum posterius behind the dorsal thickening, marks the 

 limit between mesencephalon and the primary hind-brain or 

 rhombencephalon. The beginning of the rhombencephalon is 

 also indicated by a considerable transverse extension of the 

 brain tube; posteriorly the rhombencephalon passes insensibly 

 into the spinal cord. 



We may now proceed to describe the more important events 

 in the development of each of these primary divisions of the 

 brain. We should note in advance that the prosencephalon 

 forms the olfactory lobes and cerebral hemispheres (tekncephalori) 

 and the between-brain (diencephalon) : the mesencephalon 

 forms the region of the optic lobes and chiasma: the rhomben- 

 cephalon forms the cerebellum (metencephalon) and the medulla 

 oblongata or spinal bulb (myelencephalon) . 



The Prosencephalon. The cells of the ectodermal cone 

 opposite the neuropore (Fig. 40) , soon scatter, as the tissues of 

 the head push out in advance of the brain, and no trace is 

 left of the original location of this structure, save a slight bay 

 or olfactory recess, which soon disappears. Below this level 

 the anterior wall of the fore-brain remains somewhat thick- 

 ened for a time, as the lamina terminalis. This extends to the 

 ventral side of the brain where the optic stalks extend out from 

 the fore-brain. These are hollow and their cavities are continu- 

 ous with the cavity of the prosencephalon. The regions of 

 the anterior and posterior borders of their attachment early 

 become considerably thickened as the torus transversus and 

 the rudiments of the optic chiasma and thalami, respectively 

 (Figs. 41, 42); the former becomes the seat of the anterior, and 



