THE NERVOUS SYSTEM. 475 



geminus and its central continuation, the spinal V). (3) The specialized 

 somatic sense organ, the ear, with its older vestibular and newer cochlear 

 divisions with central continuations of its nerves, including a vestibular 

 descending tract. 



These differentiations of the peripheral afferent apparatus lead to the later 

 formation of special terminal nuclei for their central continuations and second- 

 ary tracts from these nuclei to suprasegmental structures (p. 436, Fig. 371). 



The peripheral and intermediate neurones of the more highly modified 

 cranial end of the tube, or FORE-BRAIN, appear to lag behind in development, 

 but in its basal part the neuroblasts are beginning to be differentiated (fifth 

 week) . In the development of the eye, the brain wall is evaginated, carrying 

 with it the future retina comprising, apparently, the sensory epithelial cells or 

 receptors (rods and cones), the afferent peripheral neurones (bipolar cells of 

 retina) and the receptive or primary intermediate neurones (ganglion cells of 

 retina and optic nerve). The histogenesis of these elements is dealt with 

 elsewhere, but it may be pointed out here that the axones of the ganglion 

 cells of the retina grow toward the inner side of the optic cup (away from 

 the original luminal surface), pass thence in the marginal layer of the optic 

 stalk, undergo a partial ventral decussation (optic chiasma) in the floor plate, 

 and terminate in certain thalamic nuclei (lateral geniculate bodies) and in the 

 roof of the mid-brain. The so-called optic nerve is thus obviously a central, 

 secondary tract. The development of this tract does not apparently take place 

 until a later period than the differentiation of the earlier secondary tracts of the 

 cord and rhombic brain (after the sixth week) . 



In the case of the olfactory organ, it has already been seen that the peripheral 

 neurones develop at first apart from the neural tube and send their axones 

 into the olfactory bulb. The latter is an evagination of the neural tube 

 which receives the olfactory fibers, thereby constituting a complicated terminal 

 nucleus for the latter. The axones of bulb cells (the mitral cells) which pass 

 along the stalk of the bulb are thus the secondary tract of this system. Many 

 of them decussate in the anterior commissure. Secondary (and tertiary) 

 olfactory tracts find their way to caudal parts of the rhinencephalon and to 

 hypothalamus, thalamus and epithalamus, forming, with other tracts, a highly 

 modified prechordal intersegmental mechanism (p. 537). Other olfactory tracts 

 proceed to the suprasegmental archipallium which develops efferent bundles 

 to the segmental brain. 



The embryological development of the peripheral apparatus, especially 

 of its receptive portions, as shown by the various separate ganglionic rudiments 

 (Fig. 396) and placodes, exhibits a segmental character which, though not 

 in all respects primitive, is of practical value. These segments are (Adolf 

 Meyer) : (i) The olfactory apparatus, nose, without efferent elements. (2) 



