516 p. E. SMITH 



e^'agination was present in all the series of this age which I exam- 

 ined and shows but little variation, apparently having the same 

 fate in all cases. In Reptilia three paraphysial evaginations 

 have been noted (Warren '09). 



Warren ('05) in discussing the growth of the paraphysis and 

 epiphysis says : 



The roof of the fore-brain has now descended to such a degree that the 

 opening of the paraphysis is on a level with the tip of the velum. The 

 velum itself has lost its cephalic layer, and consists of one layer only, 

 which however, is much longer than the velum in fig. 5. If figs. 4, 5 and 

 7 are compared it will be seen that the distal end of the paraphysis is 

 practically at the same distance from the ectoderm in each case. As the 

 paraphysis has developed during those stages into a long tube, its growth 

 must have occurred by a downward extension of the neighborhig parts 

 into the cavity of the fore-l^rain. 



I would not agree with the author that this is the process taking 

 place. Rather than being a downward extension of the neighbor- 

 ing parts it is a dorsal extension of the parts bordering these 

 'neighboring parts.' That is, it is a growth out of the brain in 

 which the ostium of the paraphysis and the tip of the velum 

 remains practically as a fixed point. This viewpoint would 

 affect the interpretation that the cephalic layer of the velum 

 becomes the caudal layer which I do not see can be the case. 

 The telencephalon must have a dorso-caudal wall and it seems to 

 me that the simplest interpretation is that the cephalic wall of 

 the velum remains as such and that the post-velar arch is length- 

 ened by its own intrinsic growth. The post-velar arch is a 

 region of rapid growth, this growth continuing and forming the 

 diencephalic plexus. The anterior layer of the velum, likewise, is 

 a region of rapid growth, thus forming the telencephalic plexus. 

 Although the growth forming the paraphysis is at first localized, 

 later, a general growth takes place, forming the telencephalic 

 plexuses. This latter process takes place later than the for- 

 mation of the diencephalic plexus. 



At this stage the constriction separating the first and second 

 diencephalic neuromeres has disappeared. The epiphysis, dis- 

 tinctly formed, is located at the position shown in the earlier 

 stage (fig. 37) and a short distance caudad to it lies the developing 



