150 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



It is well known that in embryos of all classes of Vertebrates the 

 central nervous system shows a segmentation which consists in a series 

 of constrictions and dilatations extending throughout the length of the 

 neural tube, giving to it a beaded appearance. In the trunk the nerves 

 have a definite relation to the segments of the spinal cord, the " myelo- 

 meres," as I shall call them, adopting the term introduced by McClure 

 ('89), and it is believed that the cranial nerves also have definite re- 

 lations to the segments of the brain, the " encephalomeres," although 

 their relations are less clear. Even if we believe with Ahlbom ('84') 

 and Froriep ('94) that the nervous system is segmented in adaptation 

 to associated segmental structures, it is not a j^riori improbable that the 

 number of primitive segments in the Vertebrate may be shown by the 

 number of neural segments, for in some Invertebrate embryos segmental 

 cephalic ganglia appear even when most other traces of mesodermal 

 segments and related sense organs have (it is believed) disappeared. 



In view of the present discrepancy between the results based upon the 

 study of neuromerism and those based upon the study of mesomerism, it 

 devolves upon one who attempts to elucidate the question of cephalic 

 segmentation in Vertebrates by using the segments of the central ner- 

 vous system as criteria, to show the comparability of encephalomeres 

 with mvelomeres, not only structurally, but also in relation to ner- 

 vous outgrowths, and to those divisions of the mesoderm on which 

 the segmentation of the motor nerves ultimately depends. The inter- 

 dependence of motor nerve and muscle has seemed so evident that 

 morphologists have not hesitated to make the number of cranial nerves 

 conform with the number of somites previously determined by them. 

 Yet the majority of investigators of the segments of the encephalon have 

 failed to take into consideration the relation of these to the segments of 

 the mesoderm, and consequently we find in the literature upon neuro- 

 merism a diversity of opinion such as we have learned to expect in 

 results based upon insufficient knowledge. 



Summary of Results of former Investigations on the 

 Segmentation of the Encephalon. 

 The results of former investigators concerning the number of enceph- 

 alomeres and their nerve relations may be summarized in the form of 

 two tables. Table I. shows the number of segments as determined by 

 previous investigators, as well as their relation to the primary vesicles 

 of the brain. The total number of segments has been given in the 

 cases where it has been stated by the observer. The observations of 



