240 BULLETIN: MUSEUM OF COMPAEATIVE ZOOLOGY. 



lature). The motor fibres of nerves Y, YII, IX, and X belong to the 

 former, and nerves HI, IV, and YI to the latter class. 



While it is possible, as has been stated, to establish a numerical cor- 

 respondence of encephalomeres and somites, the nerve relations are not 

 so clear. We find, for example, that encephalomeres II, III, and VII 

 are connected by ventral (motor) nerves with somites (van Wijhe's) 

 1, 2, and 3. Such evidence of a want of segmental correspondence 

 would seem at first sight to render untenable the assumption that 

 encephalomeres have the same segmental value as myeloraeres. We 

 have already seen that these two classes of neuromeres have structurally 

 little in common. Moreover, a want of correspondence of encephalomeres 

 and visceral arches is shown by the fact that the dorsal motor fibres 

 which are connected with encephalomeres III and V innervate two suc- 

 cessive visceral arches. In view of this discrepancy in the segmental 

 relations of encephalomeres and nerves, cau we regard the former of 

 segmental value? Do they afford evidence in support of the assumption 

 that a Vertebrate head segment is comparable, i. e. homologous, with a 

 trunk segment ■? Before expressing my own opinion in regard to the 

 answer to this question I will briefly review the interpretations given 

 by previous investigators. Two antithetic views concerning the neu- 

 romeres have been given, viz. (1) that they are not of segmental or 

 phylogenetic value, and (2) that they are of phylogenetic value. 



VII. Segmental Value of Hindbrain Neuromeres. 



a. XON-PHYLOQENETIC INTERPRETATION. 



In 1877 Mihalkovics, speaking of the foldings in the medulla of birds 

 and mammals, expressed the opinion that the want of correlation be- 

 tween these structures and the nerves and visceral arches seems to favor 

 the view that they are of mechanical origin, i. e. formed by the bending 

 and shoving of the neural tube as it rapidly grows in a confined space. 

 This view seems strengthened by the consideration that the ventral wall 

 of the neural tube of chick embryos is, in early stages, markedly folded 

 into segments, irregular in size and inconstant in appearance, and that 

 these folds in the head region are visibly exaggerated by certain fixing 

 agents which result in shrinking the embryo. Balfour, who with Foster 

 ('74) had been the first to express the opinion that these structures 

 were of phylogenetic significance, afterwards ('81) said that it is uncer- 

 tain whether they have any morphological significance. In 1892 



