IV. OF RANA PIPIENS. 11 



6th. Two parts, to be homologous, must be structurally and morphologically similar, 

 and not simply the seat of similar processes. 



III. Corpora Striata. (Plate I. Fig. 7, C.) The parts supposed to be homolo- 

 gous with these ganglia are but very imperfectly developed, and present themselves 

 in the form of small nodules in the interior of the cerebral lobes, and will be again, 

 referred to in connection with the ventricles and the internal configuration of the 

 brain. 



IV. Optic Thalami. (Plate I. Figs. 1-9, D.) Behind the cerebral lobes the 

 encephalon becomes slightly contracted, and its breadth, as seen from above, is 

 formed wholly by two solid bodies, separated from each other by a distinct fissure 

 in front, but behind united by commissural fibres. (Fig. 6, D.) The fissure com- 

 municates with a cavity between them. Their position immediately behind the 

 cerebral lobes, as well as their relation to the part next to be described, to the optic 

 lobes and the optic nerves, indicates that they are the homologues of the thalami 

 optici of Mammals and Birds. 



V. Pineal Body. .(Plate I. Figs. 2, 7, 9, 11, 12, E.) This occupies its usual 

 position between the optic thalami, is situated at their anterior extremity, and almost 

 concealed by the cerebral lobes. It is a small reddish body, the presence of which 

 is not easily determined, since it is covered with a loose areolar tissue, which unites 

 it to the investing membranes, and consequently is usually torn oft' in removing 

 them. It is sustained by a membranous pedicle, but has no " peduncles." Its mi- 

 nute structure is quite different from that of the same part in the higher Vertebrates, 

 and might reasonably lead us to doubt as to its homology ; in truth, independently 

 of its position, this would not be suspected. Under the microscope it has something 

 of the form of a mulberry, and is highly vascular. Each of the rounded projec- 

 tions on its surface is supplied with a loop of a bloodvessel, all the loops being 

 derived from two or three principal trunks. (Plate I. Figs. 11, 12.) Externally it is 

 invested with a layer of ciliated epithelium. Within the epithelial layer, and ap- 

 parently among the bloodvessels, are scattered rounded cells containing granules, 

 and these bear a close resemblance to the ganglion cells of other parts of the brain. 

 This corresponds with the description of the same part in Fishes, as given by Pro- 

 fessor Owen. It is in them a constant appendage of the encephalon, " but is com- 

 monly only a vasculo-membranous pyramidal sac continued from the third ventricle. 

 Some medullary matter mingles with the membranous walls in Clupeoid and Cypri- 

 noid Fishes." * 



The existence of vibrating cilia on the surface, and of vascular loops within, are 

 points in which this body resembles the choroid plexus of the brains of Mammals 

 as described by Valentin ; but as its position is extra-ventricular, we have no author- 

 ity for identifying it with that organ. The cilia on its surface seem to indicate 

 that its function, in part at least, is to set in motion the fluids secreted by itself or 

 some of the adjoining parts. f 



* Owen, op. cit., Vol. II. p. 180. 



t Comparative Anatomy of the Pineal Body. It exists, according to Cuvier, in all Fishes, in the form 

 of a small globe of gray matter, very distinct in the common Eel and the Conger, less apparent in other 



