DEVELOPMENT OF THE NERVOUS SYSTEM. 



39 



fishes and the chick, however, they have been studied and described 

 and some of the important stages in their history are shown in the 

 accompanying series of figures. (Figs. i8, 19.) 



In the trunk region where the number of neuromeres corre- 

 sponds to the number of muscle segments and the neuromeres 

 alternate in positon with the muscle segments, the embrj^onic 

 condition is essentially continued into adult life. In the head 



V 



\ 



:f 



rf^ 



"{fr 



r/' 



B 



s. 



t'^K 



Fig. 18. — Four stages in the development of the selachian brain to show the 

 history of the neuromeres. After Locy. In A, B and C the whole head is shown, 

 in D the brain alone. The Arabic numerals indicate the neuromeres. 



region, where great changes take place during embryonic develop- 

 ment, the neuromeres share in these changes and in the adult 

 brain the embrj^onic neuromeres are lost from view. The causes 

 for this are manifold, but perhaps the chief ones are the bending 

 downward of the head, the crowding of the brain produced by its 

 growing more rapidly than surrounding organs, and above all 



