394 RtABEL BISHOP 



The brain 



Simple observation of the brain model (fig. 6) shows certain 

 conspicuous points of morphology, namely, that there is distinct 

 bilateral symmetry; that each member of the teratological brain 

 has a normal pair of cerebral hemispheres, a normal diencephalon 

 and mesencephalon; that there is a broad space between the 

 midbrains and a somewhat broader one between the forebrains 

 of the two head members; that the normality and maximum de- 

 velopment of the cranial flexures are so typical as to need no 

 comment other than to call attention to them, and that inter- 

 posed between the caudal ends of the mesencephala is the upper 

 (rostral) margin of an enlarged region of the brain which, because 

 of the sharp bend of the cephalic flexure of each head member, 

 lies dorsal to the diencephala and hemispheres, but in reality is 

 caudad to them and continuous with the spinal cord. This is 

 obviously an enlarged rhombencephalon common to both mem- 

 bers (i.e., conjoined) and therefore of special interest. 



Briefly summarizing these structural points, it may be said 

 that rostral to the cervical flexure the teratological brain shows 

 increasing duplicity and divergence, the latter being greatest at 

 the hemispheres, but in the process of doubling the primitive 

 morphological relationships and the normal embryonic flexures 

 have remained undisturbed. 



A more critical study of the model and of the microscopical 

 sections demonstrates the rhombencephalon to be considerably 

 larger than normal, as would be expected, especially in its lateral 

 dimensions, and to be associated with the principal abnormalities 

 found in the head region of the embryo. The medulla is double, 

 as indicated by a ventral forking of the central canal, which in 

 cross-section has the appearance of an inverted Y (fig. 13). At 

 the lower levels of the bulb the prongs of the Y are very shallow 

 and gradually taper into a normal spinal canal. But rostrally 

 the prongs flatten out. and become obliterated as the medullary 

 canal broadens into the spacious fourth ventricle (figs. 9 to 12). 

 Between these two levels the brain tissue included between the 

 prongs adjusts topographically to the space available. This is 



