438 



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 trabeculse. The prolongations into the lateral 

 projections of the nasal processes, which now proceed from the coalesced 

 part in question, also become but little denser in texture for the present, 

 though they elongate considerably. 



The lateral parts and the upper wall of the cranium, with the excep- 

 tion of the auditory capsules or of the subsequent bony labyrinth, remain 

 merely membranous up to the end of the second period, consisting in fact 

 only of the cutaneous covering, the dura mater, and a little interposed 

 blastema, which is hardly perceptible in the upper part, but increases in 

 the lateral walls, towards the base of the skull. 



The chorda vertebralis reaches, in very young embryos of the snake, 

 to between the auditory capsules, and further than this point it can be 

 traced neither in the snake nor in other Vertebrata, at any period of life, 

 as manifold investigations, conducted with especial reference to this 

 point, have convinced me. 



At the beginning of the third period, the basal plate chondrifies, at 

 first leaving the space beneath the middle of the cerebellum membranous j 

 but this also eventually chondrifies, and is distinguished from the rest of 

 the skull only by its thinness. 



Lateral processes grow out from the basal cartilage just in front of the 

 occipital foramen, and eventually almost meet above. They are the ex- 

 occipitals. 



The two lateral trabeculae, parts which I have also seen in frogs, 

 lizards, birds, and mammals, chondrify at the beginning of the third 

 period. At first, they pass, separate from one another throughout their 

 whole length, as far as the frontal wall, on entering which they come into 

 contact; are more separate posteriorly than anteriorly, and present, in 

 their mutual position and form, some similarity with the sides of a lyre. 

 But as the eyes increase, become rounder, and project, opposite the 

 middle of the trabeculse, downwards towards the oral cavity, the latter 

 are more and more pressed together, so that even in the third period they 

 come to be almost parallel for the greater part of their length. Anteriorly, 

 however, where they were already, at an earlier period, nearest to one 

 another, they are also pressed together by the olfactory organs (which 

 have developed at their sides to a considerable size), to such a degree, 

 that they come into contact for a great distance and then completely 

 coalesce ; they are now most remote posteriorly, where the pituitary body 

 has passed between them*, so that they seem still to embrace it. Ante- 

 riorly, between the most anterior regions of the two nasal cavities, they 

 diverge from their coalesced part as two very short, thin, processes or 

 cornua, directed upwards, and simply bent outwards. 



" It has been seen above that the median trabecula does not chondrify, 



* The pituitary body, however, as Rathke now admits, does not pass between 

 the trabeculae, and is developed in quite a different manner from that supposed in 

 the memoir on Coluber. 



