8 ON nn: M.RVOUS SYSTEM iv. 



undivided. The fusion, or rather the absence of separation, of the cerebral lobes of 

 Plagiostome Fishes, is undoubtedly to be explained in the same manner. 



It should be remembered that the olfactory lobes are not represented in the prim- 

 itive divisions of the embryonic brain, but are either the result of a transverse sub- 

 division, or perhaps of an offshoot in the form of a hernia from the cerebral lobes. 

 Bearing this in mind, we have an explanation of the union of each olfactory with 

 its corresponding cerebral lobes and with its fellow, in Frogs, which is simply the 

 result of incomplete separation or of arrested development. 



II. Cerebral Lobes. (Plate I. Figs. 1-9, B.) Agassiz, in examining the brains 

 of Fishes, has shown that those of the same natural families sometimes, and of gen- 

 era often, affect forms which to a certain extent are characteristic, and something 

 analogous to this has been observed by Blanchard to exist in the nervous sys- 

 tem of Invertebrates. A comparison of the cerebral lobes of Siren, Mcnopoma, 

 Menobranchus, Salamandra, Triton, and Rana (Plate I. Figs. 2, 4, 5), will show that 

 in them there exist, though perhaps not strongly marked, certain general character- 

 istic peculiarities, which consist mainly in having cerebral lobes with an elongated 

 form, with sides nearly parallel when seen from above, the anterior being but slightly 

 more narrow than the posterior extremity. In other Reptiles, the cerebral lobes 

 have a more triangular form, the transverse diameter being much greater behind 

 than in front. Measured on the upper surface, each cerebral lobe in the Bull-frog 

 is about two and a half times longer than broad, and the breadth of the two lobes 

 together does not exceed that of the medulla oblongata. They are laterally com- 

 pressed, so that the height of each lobe is greater than its breadth, and its greatest 

 elevation is at .its posterior part. (Plate I. Figs. 3 - 7.) They are separated by a 

 distinct fissure, which may be seen above and below, and which presents this inter- 

 esting feature, that it extends quite through from the upper to the under surface, 

 being closed in front, however, by the united olfactory lobes, and behind by the meso- 

 cerebrum ; thus presenting the opposite state of things to that which exists in 

 Rays and some Sharks, where the two are so completely fused, or, perhaps more 

 correctly, have been so little subdivided, still retaining one of their embryonic fea- 

 tures, as to have the appearance of a single lobe. This extended separation be- 

 tween the cerebral lobes, though but little noticed, certainly exists in many other 

 Vertebrates. Cuvier speaks of it in Birds, where " it is apparent that they are dis- 

 tinct throughout their whole height, and that they are united to each other behind, 

 near the anterior commissure of the brain." * Bojanus, in his admirable figures 

 of the anatomy of Testudo Europaa, represents a similar separation between the cere- 

 bral lobes, though it is not referred to in the descriptions of the plates. His Figs. 

 88 and 89 show the two hemispheres disjoined as far as the crura cerebri. Agassiz, 

 in the Anatomie des Salmones, Table V. Fig. 5, has represented a longitudinal 

 section of the brain of Salmo fario, in which it appears that the " olfactory lobes " 

 (cerebral lobes, cerebral hemispheres of other anatomists), as well as the " olfactory 

 tubercles" (olfactory lobes of others), are likewise wholly separated from each 



Cuvier, Le9ons d'Anat. Comp., 3me edit., Tom. III. p. 113. 



