444 H. V. NEAL 



The morphological and phylogenetic significance of these 

 facts is plain. Instead of being relatively young muscles which 

 have not arisen directly from the segmental muscles (as asserted 

 by Ziegler '08), and instead of heing post-otic muscles which 

 have secondarily migrated into the preotic region (as suggested 

 by McMurrich '12), the eye muscles are seen to be persistent 

 portions of the pre-otic lateral trunk musculature which owe 

 their persistence to a functional connection with the eye-ball. 

 Their separation from the lateral trunk muscles and their iso- 

 lation in the adult vertebrate are correlated with the great en- 

 largement of the ear-capsule. Their presence in the pre-otic 

 region therefore strongly suggests that in the ancestors of verte- 

 brates the myotomic segmentation extended unbroken through- 

 out the entire length of the body. Amphioxus is just such a 

 form. 



That the ancestor of vertebrates was Amphioxus-like is a 

 very generally accepted conclusion of morphologists. In this 

 connection the mesodermic segmentation discovered by Kolt- 

 zoff ('02) in Cyclostomes (Petromyzon) is especially significant 

 and important, since in this animal, according to Koltzoff, the 

 segmentation of the cephalic mesoderm is primarily total and 

 complete as in Amphioxus embryos and, moreover, the anterior 

 mesodermic segments develop, as in Amphioxus, as dorso- 

 lateral diverticula of the entoderm. Furthermore, the lumen 

 of each diverticulum is connected with the enteron just as in 

 •Amphioxus larvae. If Koltzoff' s observations be correct, Cy- 

 clostomes stand in this, as in other respects, intermediate be- 

 tween Amphioxus and Elasmobranchs. KoltzofT's observations, 

 however, have not been confirmed. 



The importance of this evidence in its bearing on the past 

 history of the vertebrate head has led me to examine sections of 

 Petromyzon embryos in those early stages before hatching in 

 which Koltzoff finds the mesodermic segmentation most clearly 

 expressed. In at least two series of sections of eight-day (Naples) 

 Petromyzon embryos the evidence presented seems to bear out 

 Koltzoff's contention that the mesodermic segmentation in Cy- 

 clostomes is comparable with that of Elasmobranchs. While 



