The two _optic thalami_, formed from the posterior and outer part of the anterior vesicle, consist at first of a single hollow sac of nervous matter, the cavity of which communicates on each side in front with that of the commencing cerebral hemispheres, and behind with that of the middle cephalic vesicle (corpora quadrigemina). Soon, however, by increased deposit taking place in their interior behind, below, and at the sides, the thalami become solid, and at the same time a cleft or fissure appears between them above, and penetrates down to the internal cavity, which continues open at the back part opposite the entrance of the Sylvian aqueduct. This cleft or fissure is the _third ventricle_. Behind, the two thalami continue united by the _posterior commissure_, which is distinguishable about the end of the third month, and also by the _peduncles of the pineal gland_.... At an early period the _optic tracts_ may be recognized as hollow prolongations from the outer part of the wall of the thalami while they are still vesicular. At the fourth month these tracts are distinctly formed. They subsequently are prolonged backwards into connection with the corpora quadrigemina. The formation of the pineal gland and pituitary body presents some of the most interesting phenomena which are connected with the development of the thalamencephalon.(677) The above is specially interesting when it is remembered that, were it not for the development of the posterior part of the cerebral hemispheres, the Pineal Gland would be perfectly visible on the removal of the parietal bones. It is very interesting also to note the obvious connection which can be traced between the originally hollow Optic Tract and the Eyes anteriorly, and the Pineal Gland and its Peduncles posteriorly, and between all of these and the Optic Thalami. So that the recent discoveries in connection with the third eye of Hatteria Punctata have a very important bearing on the history of the development of the human senses, and on the Occult assertions in the text. It is well known that Descartes saw in the Pineal Gland the _Seat of the Soul_, though this is now regarded as a fiction by those who have ceased to believe in the existence of an immortal principle in man. Although the Soul is joined to every part of the body, he said, there is one special portion of the latter in which the Soul exercises its functions more specially than in any other. And, as neither the heart, nor yet the brain could be that “special” locality, he concluded that it was that little gland which was tied to the brain, and yet had an action independent of it, as it could easily be put into a kind of swinging motion “by the _animal spirits_(678) which cross the cavities of the skull in every sense.”