THE SKULLS OF REPTTLIA AND AVES. 239 



this has happened, the fold diminishes in height and eventually 

 vanishes, almost completely. 



The two lateral trabecule, 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 thick- 

 ness, 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, how- 

 ever, becoming so dense as the hinder, longer part of the tra- 

 becule. The prolongations into the lateral projections of the 

 nasal processes, which now r 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 exception 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 notochord 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 Verte- 

 hrata, 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 ; 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. 



