236 D. DAVIDSON BLACK 



In cyclopian mammals no mechanical difficulties incompatible 

 with life are met until after birth. The animal then usually 

 dies in a short time from interference with feeding and respiratory 

 functions. 



Thus, bearing in mind that the cosmobion is governed in all 

 its growth changes bj' quite definite mechanical laws, an attempt 

 may be made to interpret the form relations in this case. Certain 

 areas have been practically absent during development. If one 

 removed such areas from a model of a very young normal brain 

 and approximated the cut edges, would the resulting malforma- 

 tion be similar to the case in hand? 



If one examine one of His' models of the brain at the end of 

 the fourth week, it will be seen that the telencephalon is rep- 

 resented by an expanded, thin walled unpaired vesicle, for at 

 this stage the median furrow between the two pallial expansions 

 is not developed. Looking at a mesial sagittal section through 

 such a model, it is possible to mark out on the ventricular 

 surface, with a considerable degree of accuracy, the areas which 

 will later be developed into the corpus striatum and rhinen- 

 cephalon, the pars optica and the pars mammillaris hypothalami, 

 and the pallium (fig. 21). 



It has been shown that in the present case, the pallium alone 

 of these parts is present. Thus, if one cuts out from both sides 

 of a clay model of the brain at the end of the fourth week, these 

 areas which are not developed, and places the two halves remain- 

 ing in apposition, then by simply pressing the cut surfaces of 

 each half together one has reproduced in all its essentials the 

 form relations obtaining in this cyclopian brain (fig. 22). It must 

 be borne in mind, however, that this is reversing the true sequence 

 of events. 



We are left with a cup whose thickened walls are formed by 

 all the pallium, and whose rim is formed by the two original me- 

 sial edges, to which of course is attached the much stretched 

 thin roof. These mesial edges in the normal pallium for the 

 most part are the areas in which the hippocampal formations 

 are laid down. It is also along these mesial edges that the lateral 

 choroid plexuses are normall}'^ invaginated. So in this case, we 



