256 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



the loss of the somatic musculature of this segment, a somatic (ventral) 

 nerve is wanting, and as in the case of the 6th segment I assume that 

 this is to-day repi'esented by the abducens. Since the sensor fibres of 

 the f;\cialis innervate the skin of the hyomandibular (1st visceral) cleft, 

 this cleft may be believed to have been primitively situated ventral to 

 the constriction betAveen the 3d and -ith somites. I find no evidence to 

 support the view that the hyoid arch represents two splanchnic segments. 



d. Relations of Encephalomere IV. 



As has already been stated, there is no ganglionic nerve Aulage 

 proliferated from encephalomere IV, and although the fibres of the major 

 root of the trigeminiis have their exit in early stages from its outer con- 

 vexity, the probability is that such relation of nerve V is secondary, and 

 therefore not of phylogenetic significance. I hold that this encepha- 

 lomere, by virtue of its local thickening, aflfords evidence of a lost visceral 

 arch, the loss of which would naturally be correlated with the loss of the 

 dorsal nerve. Since, however, the disappearance of tlie splanchnic por- 

 tion of this segment may have been due simply to a union with the 

 corresponding portion of the anterior (mandibular) segment, it is also 

 possible that the dorsal nerve has fused with the nerve of the anterior 

 segment, the trigeminus. This conclusion seems indeed supported by 

 the evidence that at least some of the fibres of the trigeminus roots have 

 their nucleus in the lateral horn of this (-ith) encephalomere. 



In a scheme of primitive segmental relations such as I am at present 

 advocating, there is likewise difficulty in explaining the fact that the 

 somite (van Wijhe's 3d) which I assume to corresjjond with encephalo- 

 mere IV is innervated by the abducens, whose fibres make their exit 

 from encephalomere VII. The evidence which leads me to conclude 

 that the abducens to-day represents the primitive ventral nerve of this 

 encephalomere, as well as those of encephalomeres V, VI, and VII, has 

 been partly given in connection with the study of its development ; it 

 may be summarized as follows. (1) Its roots are many (4-6 in various 

 Selachii) and more widely separated than those of any other nerve. 

 (2) jSTot only do abducens fibres innervate pre-otic musculature (muse, 

 rectus posterior), but fibres from this nerve may also be ti'aced for a con- 

 siderable distance in the mesoderm of the embryo posterior to encepha- 

 lomere VII (Fig. 20). (3) The variability as to the place where its fibres 

 emerge, as shown by comparative embryological evidence, appears to 

 indicate that its relations are not limited to any single encephalomere. 

 (4) Its nucleus in the ventral horn of the neural tube is greatly elongated. 



