124 BIOLOGICAL LECTURES. 



" Furthermore, the vagrant changes in the position of some 

 of the cranial nerves must necessarily cause confusion. For 

 example, take the sixth nerve, which in the frog and tadpole 

 stages is situated between the first and second roots of the 

 ninth nerve (given on the authority of Dr. Strong), a position 

 somewhat posterior to its place of origin. This remarkable 

 shifting clearly shows not only what great changes in position 

 the cranial nerves are capable of undergoing, but it also goes 

 to prove that we can find no reliable means of determining the 

 primitive segments by means of their connection with the exit 

 of the existing cranial nerves. Beard, in taking up this problem, 

 made use of an important series of sense-organs for which he 

 proposed the name ' branchial sense-organs,' from their de- 

 velopment from thickenings of the epiblast over each branchial 

 cleft. The dorsal branches of certain cranial nerves fuse with 

 these epiblastic thickenings ; the superficial part of the thicken- 

 ing gives rise to a branchial sense-organ, while the deeper por- 

 tion becomes the ganglion of the dorsal root of the cranial 

 nerve. This close relation which exists between the dorsal 

 branches of the cranial nerves and their corresponding sense- 

 organs is undoubtedly of segmental character. But this line of 

 research is beset by a great difficulty, namely, that the degen- 

 eration of certain sense-organs would, in time, involve the 

 degeneration of their corresponding cranial nerves, and such 

 degeneration has taken place, in part or in whole, leaving in 

 doubt the primitive segments with which they were connected." 



The mesoblastic head-cavities and neural segments are both 

 more important clues to the metamerism of the head. The 

 mesoblastic head-cavities are called myotomes, and embody 

 the muscle rudiments, while the neural segments represent the 

 joints of the nervous system. Muscle and nerve are, physio- 

 logically, so fundamentally related that we should naturally ex- 

 pect some close correspondence between muscle segments and 

 neural segments, and metamerism of the head should be studied 

 in the light of observations on both sets of structures. 



Balfour first studied the segmental divisions of the meso- 

 blast in the head of elasmobranch fishes, and, in 1874, identi- 

 fied by this means eight head somites. He also expressed the 



