MORPHOLOGY OF EYE MUSCLE NERVES 135 



In the midbrain as many as eight may be reckoned, and in other 

 eml)ryos none at all. A s^^mmetrical number sometimes occurs. In 

 the cases just examined, I find five specimens out of fourteen showing 

 the same number of midbrain neuromeres on both sides. In one case 

 I counted three right, three left; in another eight right, eight left; 

 in another three right, three left; in another four right, four left, and 

 finally one in Avhich but a single neuromere appeared on either side. 

 In the hindbrain, the number of neuromeres varies between three and 

 twenty-four, and they differ in number on different sides, a difference 

 of ten having Ijeen noted in the same individual in right and left sides 

 of the body. 



It may be objected of course, (1) That in these cases of asymmetry 

 the neuromeres were present on the 'off' side but were 'suppressed,' 

 and therefore could not be counted. (2) That the change in the number 

 of neuromeres might be due to different stages in development, the 

 neuromeres appearing most completely at a definite date of development. 

 The fact remains, nevertheless, that asymmetry and variable numbers 

 are present, and to an extraordinary degree, a state of affairs which 

 does not make in the dii'ection of clearing up our knowledge of these 

 'segmental' structures. (3) That the neuromeres of Bdellostoma 

 are artifacts. Against this criticism I note that I have seen them clearly 

 in living embryos. 



But it may be objected that the foldings of the Bdellostoma 

 brain are not the sjrmmetrical foldings which have been recog- 

 nized as neuromeres in other forms; that no one would be likely to 

 interpret the variable and asymmetrical structures which Dean 

 describes in Bdellostoma as of phylogenetic or morphological 

 value; that real neuromeres are symmetrical and permanent 

 thickenings of the brain wall. In reply to such objections it 

 should be remembered that morphologists have not always in- 

 sisted upon the constancy or the symmetry of problematic struc- 

 tures. In fact Dohrn ('04), in reply to this objection to his 

 microcoelic mesodermic segments, replied that inconstancy and 

 asymmetry would be expected in degenerating metameric struc- 

 tures like these. Out of hundreds of Squalus embryos examined 

 by the writer in order to confirm Locy's results only two or three 

 showed symmetry or regularity in the segmentation of the edges 

 of the neural plate (figs. 2 and 3, Neal '98). Yet morphologists 

 have not refused to accept Locy's conclusions because of this 

 lack of confirmation of his results on selachians. 



