ON THE THEORY OF THE VERTEBRATE SKULL S^9 



about the middle of their length, separating as they passed forwards ; afterwards, 

 they converged, so that, at their extremities, they were separated by a very small 

 space, or even came into contact. Altogether they formed, as it were, two horns, 

 into which the investing mass of the chorda was continued forwards. The 

 elongated space between them, moderately wide in the middle, was occupied by 

 a layer of softer formative substance, which was very thin posteriorly, but some- 

 what thicker anteriorly. Upon this layer rested the infundibulum ; and in front 

 of it, partly on this layer, partly on the trabecute, that division of the brain 

 whence the optic nerves proceed, and further forwards the hemispheres of the 

 cerebrum. Anteriorly, both trabeculse reached as far as the anterior end of the 

 head, and here bent slightly upwards, so that they projected a little into the frontal 

 wall of the head, their ends lying in front of the cerebrum. Almost at the end of 

 each horn, however, I saw a small process, its immediate prolongation, pass out- 

 wards and form, as it were, the nucleus for a small lateral projection of the nasal 

 process of the frontal wall. 



The middle trabecula grows, with the brain, further and further into the cranial 

 cavity, and as the dura mater begins to be now distinguishable, it becomes more 

 readily obvious than before, that the middle trabecula raises up a transverse fold 

 of it, which traverses the cranial cavity transversely.^ The fold itself passes 

 laterally into the cranial wall ; it is highest in the middle, where it encloses the 

 median trabecula, and becomes lower externally, where it forms, as it were, a 

 short ala proceeding from the trabecula. With increasing elongation, the trabecula 

 becomes broader and broader towards its free end, and, for a short time, its thick- 

 ness increases. After this, however, it gradually becomes thinner, Avithout any 

 change in its tissue, till, at the end of the second period, it is only a thin lamella, , 

 and after a short time (in the third period) entirely disappears. 



In mammals, birds, and lizards, that is, in those animals in general, in which 

 the middle cerebral vesicle is very strongly bent up and forms a protuberance, 

 while the base of the brain exhibits a deep fold between the infundibulum and 

 the posterior cerebral vesicle, a similar part to this median trabecula of the skull is 

 found. 



In these animals, also, at a certain very early period of embryonic life, it 

 elevates a fold of the dura mater which passes from one future petrous bone to the 

 other, and after a certain time projects strongly into the cranial cavity. Some- 

 what later, however, it diminishes in height and thickness, as I have especially 

 observed in embryos of the pig and fowl, until at last it disappears entirely 

 in these higher animals also, the two layers of the fold which it had raised up 

 coming into contact. When this has happened, the fold diminishes in height and 

 eventually vanishes, almost completely. 



The two lateral trabeculae, which in the snake help to form the anterior half 

 of the basis of the skull, attain a greater solidity in the second period, acquire a 

 greater distinctness from the surrounding parts, and assume a more determinate 

 form becoming, in fact, filiform, so that the further forward, the thinner they 

 appear. They increase only very little in thickness, but far more in length, during 

 the growth of the head. Altogether anteriorly, they coalesce with one another, , 

 forming a part which lies between the two olfactory organs and constitutes a 

 septum. As soon as these organs increase markedly in size, this part is 

 moderately elongated and thickened, without however becoming so dense as the 

 hinder longer part of the trabeculae. The prolongations into the lateral projections, 



1 What Rathke terms the 'middle trabecula,' appears to be only very indistinctly 

 developed in Fishes and Amphibia. 



