408 MABEL BISHOP 



Superjicial origin and peripheral distrihution of the nerves of the 

 7nedian {conjoined) region. The relationship of structures in 

 this region, due to the juxtaposition of the head moieties, should 

 not be forgotten. The identification of at least a few structural 

 landmarks is an essential guide to nerve identification, and these 

 are specially considered in relation to nerve VII. 



In dealing with teratology from a very different viewpoint, 

 Wilder ('08) discussed the nerves concerned with the musculature 

 of the median eyes of this embryo (Teras I) and hazarded an 

 interpretation of some others. Bis investigation in the latter 

 regard was admittedly less detailed than the present study, and 

 while agreeing with many of his findings, others are amplified, 

 and still others are at variance, as may be noted by a comparison 

 of the following description with that of Wilder ('08 pp. 412- 

 417, fig. 29). 



The present study of the median region has been made by 

 careful comparison with known structures in the outer (normal) 

 moiety of each head, both regions again rechecked by normal 

 embryos, and the cartilages further verified by a wax-plate model 

 of the chondrocranium of the teras and of a normal embryo 

 slightly older. 



Nerves one, two, three, and four are independent of fusion or 

 doubling and may be dismissed without further comment, since 

 they are merely normal mates of those on the outer side of each 

 head, respectively. The real problem begins, therefore, with 

 the trigeminus and involves all of the cranial nerves caudad to 

 it. Figure 5 represents a median sagittal section through the 

 model and shows with considerable clearness the relationship 

 of the median nerves to each other and to the brain. 



Earlier in the paper it was pointed out that there is a neural 

 ridge along the median ventral surface of the spinal cord, which 

 gives rise to median (conjoined) spinal nerves. Attention is 

 now called to two median cranial ridge-like masses along the 

 ventral surface of the medulla, which are separated from each 

 other and from the spinal ridge by a short hiatus (figs. 6, 5). 

 These cranial ridge-like masses are not evaginations of the brain 

 wall, as the spinal ridge is an evagination of the spinal cord, 



