38 NERVOUS SYSTEM OF VERTEBRATES. 



tance. The second is that in front of the mesencephalon the crest 

 is either very small or so modified that it can not be readily com- 

 pared with that of the trunk. 



While these changes are taking place two special sense organs 

 are making their appearance, the olfactory and auditory organs. 

 Both are formed from parts of the ectoderm immediately adjacent 

 to the neural plate. The olfactory organ appears as a pair of thick- 

 ened patches of ectoderm at the cephalic border of the neural plate 

 at either side of the neuropore. These patches eventually become 

 depressed and form deep olfactory pits. The auditory organs 

 arise on the dorso-lateral surface of the head opposite the region 

 of the future myelencephalon. These are also at first thickened 

 patches which sink in and form deep pits, which eventually separate 



FIG. 17. A part of the spinal cord of an 1 8-day Catostomus embryo showing 

 two giant ganglion cells. Golgi method. 



from the ectoderm as closed sacs and may press against the neural 

 tube on either side. These auditory sacs exercise a great influence 

 on the form and course of development of other organs. 



If now the neural tube be examined either in frontal sections 

 or in embryos so dissected as to lay the tube bare, it will be seen 

 that the tube is divided by slight transverse constrictions into 

 successive segments approximately equal in length. These seg- 

 ments are called neuromeres. The neural crest is divided into 

 corresponding segments and, as shown in Fig. 18, the segmentation 

 of the crest is present in the neural plate stage in selachian embryos. 

 While these neuromeres have been clearly seen and described 

 in the region of the hindbrain and spinal cord in all classes of 

 vertebrates, in the anterior portions of the brain they are of shorter 

 duration and are more difficult to study. In selachians, bony 



