437 



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 somewhat 

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

 front of it, partly on this layer, partly on the trabeculae, that division 

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

 hemispheres of the cerebrum. Anteriorly, both trabeculee 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 outwards 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 trans- 

 versely*. 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 thickness in- 

 creases. After this, how r ever, it gradually becomes thinner, without 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. Somewhat 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 trabeculse, 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 



* What Rathke terms the ' middle trabecula,' appears to be only very indi- 

 stinctly developed in Fishes and Amphibia. 



