DEVELOPMENT OF THE BEAIN AND GANGLIA. 59 



and become bipolar or pseudo-unipolar ganglion-cells, while the cells of the 

 fundament -which remain superficial become differentiated into "secondary 

 sense-cells" of Eetzius (Froriep). 



While the relations in the trunk are quite simple,— since here a series 

 of spinal ganglia lie beside the spinal cord, — those in the head will be 

 found to present a much more complicated arrangement. It is exceedingly 

 important here to obtain a clearer idea as to the number of fundaments 

 or primitive elements which enter into these structures, because in all 

 adult vertebrates of higher rank the relations presented by the cranial nerves 

 are so complicated that the opinion has long been held that these nerves are 

 not completely homologous to spinal nerves, but that all or nearly all of 

 them are compounded from several elements into an apparently unified 

 structure. Twelve pairs of cranial nerves are usually differentiated in mam- 

 mals; even in birds the same thing may be true; but in amphibians and espe- 

 cially in fishes such an enumeration is quite arbitrary. In these lower verte- 

 brates not only do the nerve-trunks receive other branches than those typical 

 of the higher animals, but the sources of the roots are so manifold that it 

 is not always possible to determine whether a particular root belongs to one 

 nerve or another. For example, the Facialis consists essentially of motor 

 fibers in the higher vertebrates, but in the aquatic lower vertebrates it re- 

 ceives a large reinforcement of sensory fibers which belong to the system of 

 cutaneous sense-organs. Thus there exists no little confusion in differen- 

 tiation of the Facialis and Trigeminus. 



The cephalic end of the skeleton is phylogenetically compounded from 

 a number of segments which have not yet with certainty been determined. 

 It is probable that the number of ganglia and the nature of the nerve-roots 

 may serve as guides in this problem. It thus becomes an important task to 

 determine: (1) how many primitive pairs of cranial nerves there are, and (2) 

 how these have been transformed and combined to form the cranial nerves 

 which we find in the higher vertebrates. Incident to the transfo:?mation 

 certain previously large and important nerves become superfluous and dis- 

 appear, others change the direction of their course, and still others pass to 

 organs which, during the racial or individual development, have had a 

 function quite different from that which they have in the mature condition. 

 For example, from the embryonic branchial fundaments important parts of 

 the skull and of the middle ear develop; the branchial nerves are modified 

 along with the other structures, and appear later in a role wholly different 

 from that which they originally played (e.g., N. petrosus superfic). In fishes 

 and aquatic amphibians the skin of the head is covered with a system of 

 sensory end-organs which possibly serve for the perception of the changes 

 of pressure in the surroimding medium. The sensory branches of the 

 Facialis which in Amphibia innervate these organs are, in the metamor- 



