148 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



In its development, as stated above, the brain is, from the first, 

 larger than the cord. Very early the brain tube develops three en- 

 largements, separated from each other by two constrictions, the 

 third of these enlargements passing gradually into the cord (fig. 

 155, i) . These enlargements are called, from the front end backward, 

 the fore-brain, the mid-brain and the hind-brain, the constriction 

 between mid- and hind-brains being the isthmus. From the anterior 

 end a groove, the limiting sulcus (S. of Monro), may run along the 

 middle of the inner surface of the side walls of the tube, dividing it 

 into a dorsal and a ventral zone. In front, the limiting sulcus ends 

 in a small pit, the optic recess, wedged in between the two zones of 

 either side, and the part of the roof plate immediately above the recess 

 retains its non-nervous character through life and is known as the 

 lamina terminalis. 



This distinction of the two zones is of great importance, for the 

 dorsal is to be connected with the sensory structures, the ventral will 

 be motor in character. Also, the most noticeable changes in develop- 

 ment, as well as in the evolution of the vertebrate brain, are concerned 

 with the dorsal zone, the ventral being much more conservative. 

 As in the cord, the side walls are the chief seat of nervous develop- 

 ment, while roof and floor plates, for the most part, retain their 

 non-nervous character, though in some places they may be invaded 

 by nervous matter from the sides. 



The dorsal zones of the two sides of the fore-brain give rise to sev- 

 eral structures. The anterior part on either side grows out laterally 

 and anteriorly (fig. 155, 2), the outgrowths increasing rapidly in 

 size, eventually forming a pair of hollow vesicles, the telencephalon 

 (cerebrum, cerebral hemispheres) with the lamina terminalis at the 

 bottom of the interval (longitudinal fissure) between them (fig. 

 157, /). In the wall of each hemisphere is a basal ganglionic portion, 

 the corpus striatmn, and an anterior area which grows out to join the 

 ectoderm of the front of the head where the olfactory organs 

 are to form, this part being the olfactory lobe (rhinencephalon) . 

 The remainder of the wall of each vesicle forms the paUiimi or 

 mantle. 



The roof plate of the telencephalon does not become nervous, but 

 from it are developed a thin-walled sac, th e paraphya s, which extends 

 outward from the brain, and behind this, an infolding of the roof as 

 a velum transversum, usually recognized as limiting the cerebrum 

 behind (fig. 158). 



