430 AN INTRODUCTION TO ZOOLOGY 



The posterior division of the brain- is the myelencephalon. It 

 includes the seventh to the eleventh neuromeres, and its roof remains 

 very thin, non-nervous, and is finally transformed into the choroid 

 plexus of the fourth ventricle. Its walls and floor thicken enormously 

 later on to constitute the medulla oblongata. 



Behind the hind-brain the neural canal continues backwards 

 and shows practically no signs of segmentation. Even from the 

 first its sides are thicker than its roof and floor, so that the lumen is 

 elongated in the dorso-ventral direction. During the first three days 

 of incubation the walls increase slightly in thickness, but the chief 

 development is to be found in the cells composing them, which have 

 differentiated into two varieties : epithelial cells lining the tube and 

 stretching to its outer limits, and more rounded germinal cells 

 occupying the interstices between them. From the former come the 

 epithelial cells forming the lining of the central canal, and known as 

 the ependyma. The germinal cells, on the other hand, provide the 

 actual nerve cells constituting the grey matter of the spinal cord, 

 and these neuroblasts, as they are termed, develop into typical 

 ganglion cells. 



Sense Organs, 



The three main organs of special senses, taking them in the order 

 in which they appear, are the eye, the ear and the olfactory organs. 



Eye. : . , 



Early in the second day, before the neural folds have met, the 

 lower side walls of the fore-brain region show distinct outbulgings. 

 These are more marked upon the closure of the folds, and, as has 

 been noted above, the aperture between them and the fore-brain 

 cavity becomes considerably constricted, forming at last a tubular 

 optic stalk. The distal portion is dilated to form the primary optic 

 vesicle, and its outer wall almost touches the ectoderm, which com- 

 mences to thicken in this region at quite an early stage, giving rise 

 to the lens rudiment. The outer and ventral wall of the vesicle 

 also thickens and sinks inwards towards the inner wall. Simul- 

 taneously with this invagination process the rudiment of the lens 

 also sinks in to form a thick-walled depression that at first almost 

 fills the inside of the cavity on the outside of the optic vesicle. The 

 invagination continues until the thicker originally outer wall comes 

 to lie close against the thinner original mesial wall, and so the cavity 

 of the vesicle is obliterated. In this way a double-walled optic cup 

 or secondary optic vesicle is formed, whose cavity represents the 

 posterior or vitreous chamber of the eye. At the same time the 

 lens invagination gets deepei, becoming transformed into a relatively 



