PARAPHYSIS AND PINEAL REGION IN REPTILIA 323 



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velar arch so that the arch itself has been reduced to a narrow 

 pocket while its roof has been folded transversely. The trans- 

 verse folds have been invaded by the overlying mass of vessels 

 and we have now the first appearance of a choroid plexus develop- 

 ing from the diencephalon, D. C. P. This diencephalic plexus 

 is therefore of later origin than the telencephalic, which first 

 appeared at the stage shown in fig. 10, 17.5 mm. The epiphysis, 

 like the paraphysis, has elongated considerably and lies close 

 against the wall of the post velar arch. It is still hollow, but its 

 cavity no longer communicates with that of the diencephalon. 

 It is, however, attached to the roof of the brain by a solid con- 

 stricted stalk which separates the two commissures. The superior 

 commissure is rather larger than in the previous stage, while the 

 posterior now consists of two very distinct parts, — the posterior 

 being bent forward over the anterior at a very acute angle. This 

 is also due to the forward development of the roof of the hind 

 brain. In the lateral wall of the diencephalon is seen in section 

 the so-called middle commissure, which contains a distinct cav- 

 ity. There is also a deep pocket extending down into the floor 

 of the brain immediately above the optic commissure. The 

 velum transversum is clearly seen just behind the lower end of the 

 paraphysis. It is much thickened and contains a band of commis- 

 sural fibers passing between the mesial walls of each hemisphere 

 which is the aberrant commissure described by Elliot Smith. 

 The plexus of the lateral ventricles has become more convoluted, 

 its greatest extent being still in the vertical diameter. The incli- 

 nation of the cavity of the hemispheres downward and forward is 

 much more marked than in earlier stages. This ventral bending, 

 together with the peculiar development of the mid brain contrib- 

 utes to the closing up of the post velar arch and the reduction of 

 the upper part of the diencephalon to a cavity very limited in its 

 antero-posterior diameter but relatively broad transversely. 



Fig. 13 is from a model of the pineal region of a brain of an 

 adult Lacerta muralis. The lower outline of the model is very 

 irregular as it was impossible to get all the sections into the field. 

 The paraphysis is very striking, being a long tube rather narrow 

 in its lower half but increasing in size in its upper half from which 



