536 



that, because the swollen mass of rudimentary hippocampus, which is 

 seen in the foetal mammal, aöords attachment to the lamina chorioidea, 

 the prominent object (Fig. 18, P), which presents exactly similar re- 

 lations in the foetal reptile, must also be the homologue of the hippo- 

 campus. That this solution of the problem is quite erroneous I have 

 conclusively demonstrated in my memoir on the cerebral commissures 

 of Sphenodon (op. cit.. Trans. Linn. Soc, see especially Figs. 12 to 19). 



But if the mass P (Fig. 18) is not the hippocampus, how can we 

 explain the apparent want of harmony between these different verte- 

 brates, so far as the constitution of the mesial walls of their hemi- 

 spheres is concerned? The full consideration of this matter would 

 demand a large number of illustrations to elucidate the account, and, 

 moreover, would carry us far beyond the limits imposed on this dis- 

 cussion at present. I may briefly indicate the conclusions which the 

 study of all the facts of the case seems to suggest. That the caudal 

 extension of the paraterminal body is an anomaly peculiar to certain 

 reptiles cannot be maintained. That the whole, or, in fact, any con- 

 siderable part, of such a (nervous) prolongation of the paraterminal body 

 exists as the matrix of the fimbria (fornix) in the Mammalia is also 

 not the case. There is only one other alternative hypothesis, namely, 

 that the caudal part of the paraterminal body (Fig. 18, P), which is 

 present in certain reptiles, becomes reduced in other animals to an 

 epithelial lamella in much the same manner as we have seen it behave 

 in Lepidosiren, as the plump mass P in Fig. 15 was followed back- 

 ward in Figures 16 and 17. This hypothesis fits all the facts at present 

 known for vertebrates in general: moreover, it opens the way for an 

 explanation of the enigmatical state of affairs found in the brain of 

 Ceratodus. 



There are two common ways of regarding the derivation of the 

 choroid plexuses of the lateral ventricles. Those who still believe in 

 the existence of a primary undivided "Telencephalon", which becomes 

 secondarily subdivided into the two cerebral hemispheres, naturally 

 look upon the choroid plexus as an invaginated, thinned portion of the 

 wall of the hemisphere, and speak of a "choroidal fissure" as a feature 

 of the hemisphere itself. Most of those who believe that the hemi- 

 spheres are paired from the first, arising independently as lateral buds 

 from the forebrain-vesicle, look upon the lateral choroid plexus as a 

 derivative of the roof of the forebrain and not as a part of the hemi- 

 sphere: such anatomists do not include the so-called "choroidal fissure" 

 among the furrows of the cerebral hemisphere. 



As a believer in the latter way of looking at the facts I regarded 



i 



