272 bulletin: museum of comparative zoology. 



truthfully said that cranial nerves differ from spinal in that the former 

 extend laterad and the latter mediad of the mesomeres. We must con- 

 clude that dorsal nerves were in all probability, as in Amphioxus, re- 

 lated to the septa between myotomes. Finally, the distinction made by 

 His, in the case of dorsal cranial nerves, between dorsal (sensor) and 

 lateral (motor) routs, has, with the knowledge of the facts above stated, 

 an anatomical and physiological rather than a morphological interest. 

 I therefore see no escape from the conclusion that the occipital region of 

 the head is not a region sui generis, and I pass to the consideration of 

 the pre-occipital segments. 



To those who are deeply impressed with the differences between post- 

 otic and pre-otic regions of the Vertebrate head, it is necessary to em- 

 phasize the following fundamental resemblances in the segments of these 

 two regions. (1) Pre-otic and post-otic encephalomeres have been 

 shown to be morphologically comparable. (2) The dorsal nerves con- 

 nected with these, and (3) the visceral arches which these nerves supply 

 are in these two regions serially homologous. Moreover, as evidence 

 pointing in the same direction, it may be stated that (4) a post-otic 

 nerve innervates pre-otic musculature. Furthermore, the serial homol- 

 ogy of pre-otic and post- otic somites appears established by the fact 

 that (5) a pre-otic somite (van Wijhe's 3d somite) is a segment of the 

 dorsal mesoderm. That it is such seems clear, for it is defined 

 anteriorly and posteriorly by well marked constrictions (observed by 

 several investigators), it becomes differentiated into myotome and sclero- 

 tome, and its musculature appears first in its median wall, and becomes 

 innervated by a ventral nerve (abducens) serially homologous with ven- 

 tral spinal nerves. The fact that the primitively dorsal mesoderm of the 

 pre-otic region grows ventrally to form the splanchnic musculature, as 

 has been stated for Cyclostomes, Selachii, and Amphibia, is not a basis 

 for a fundamental distinction between post-otic and pre-otic regions, 

 since this is the method of formation of splanchnic mesoderm through- 

 out the length of the body in Amphioxus. In this respect, as in 

 respect to the nerves, the head shows more primitive conditions than 

 the trunk. Since the literature of the last decade and a half shows 

 little agreement of opinion as to the morphology of the eye-muscle 

 nerves, more especially the oculomotorius and the trochlearis, and 

 since in the preceding pages evidence has been given which tends to 

 reconcile existing differences, it is important to consider briefly the 

 bearing of their morphology upon that of the pre-otic segments. The 

 more recent attempts to classify the eye-muscle nerves as dorsal, 



