36 McCLURE. [Vol. IV. 



use of as a means of determining the original number of 

 segments. 



2. Furthermore, the vagrant changes in the position of some 

 of the cranial nerves must necessarily cause confusion. For 

 example, take the VI nerve which in the frog and tadpole stages 

 is situated between the first and second roots of the IX. nerve,^ a 

 position somewhat posterior to its place of origin. This remark- 

 able 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 

 has proposed the name of "Branchial Sense Organs," from their 

 development from thickenings of the epiblast over each bran- 

 chial cleft. The dorsal branches of certain cranial nerves fuse 

 with these epiblastic thickenings ; the superficial part of the 

 thickening giving rise to a branchial sense organ, while the 

 deeper portion 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 

 degeneration of certain branchial sense organs would, in time, 

 involve the degeneration of their corresponding cranial nerves, 

 and such degeneration has certainly taken place, in part or in 

 whole, leaving in doubt the primitive segments with which they 

 were connected. As far as I have been able to compare Beard's 

 investigations with my own, I think they are correct when he 

 considers the I., III., V., VII., VIII., IX., and X nerves in 

 connection with their corresponding branchial sense organs, as 

 representing respectively the remains of primitive segments. 

 In fact, my own observations lead me to the same conclusions, 

 but in addition to these I find intermediate encephalic segments 

 between I. and VIII. nerves which Beard's method has led him 

 to pass over entirely. 



These investigations were carried on in the Morphological 

 Laboratory of Princeton, under the direction of Dr. Henry F. 



1 I am indebted to Mr. Strong of Princeton for this point. 



