17° Journal of Comparative Neurology. 



(ii) The isthmus is longer than any of the so called 

 neuromeres. 



{12) The trigeminal neuromere is well and early developed. 

 The roots of the nerve pass both cephalad and caudad into 

 adjoining neuromeres. It is possible to regard it as represent- 

 ing, with the third and sixth nerves, three primary neuromeres. 



(13) The fourth hind-brain neuromere is no larger than the 

 seventh and eighth. 



(14) The fifth and sixth are nearly equal and give rise to 

 the ninth and tenth in a similar way. Tl.e eleventh is seggre- 

 gated at a late period. 



(15) The cord is distinctly segmented at one period. This 

 segmentation is of a sort inexplicable by appeal to mechanical 

 effects of the metamerism of the protovertebrae. 



(16) The neuromeres of the medulla cannot be ascribed to 

 the mechanical effects of the anlags of the nerves, for those 

 segments which have no nerves develop equally with the others. 



(17) The auditory sac and ganglion and other structures of 

 this region do not enhance but rather suppress the neuromeres. 



It is hoped soon to present the evidence more fully than is 

 now possible. 



The youngest stages of Eutaenia at out disposal are repre- 

 sented by a somewhat oblique series about parallel with the 

 middle part of the medulla. The embryos are figured on Plate 

 XVI, Figs. 1,2. The sections, Plate XVIII, Figs. 5 — 9, plainly 

 show that the medulla is distinctly segmental at this early stage 

 and that the segments have a definite relation to the cranial 

 nerves. There are five such expansions beginning with the 

 trigeminal which is larger ( though not so obviously as later) 

 than the rest. In spite of the great flexures of the mesencepha- 

 lon it is obviously composed of at least two parts. 



Similar sections parallel to the axis of the mid-back region 

 show that, at this stage, the neuromensm is confined to the 

 medulla (See Fig. 12). In the oldest specimens seen the same 

 relations prevail except that the cephalic parts are exagerated. 

 Plate XVIII, Figs, i — 4, represented four sect ons nearly 

 parallel with the medulla. The embryo from which these were 

 taken is figured Plate XVI, Figs. 4 — 5. 



