DEVELOPMENT OF THE CHOROID PLEXUS 99 



DISCUSSION 



1. Telencephalon 



a. Recessus preopticus. In all three embryos there is no 

 doubt about the identity of this recess. 



b. Velum iransversum. Just as obvious is the location of the 

 velum transversum. It is marked by the groove running across 

 the roof plate joining the anterior ends of attachment of the 

 tela chorioidea diencephali to the thalamus. At the lateral 

 end of the velum transversum the taenia thalami meets the 

 taenia chorioidea ffig. 26) and at this point the velum trans- 

 versum becomes continuous with the angle between the taenia 

 chorioidea and the lateral thalamic wall. In figure 26 an arrow 

 lies in the angle between the taenia chorioidea and the lateral 

 thalamic wall and continues in the velum transversum. The 

 position of the head of the arrow in the mid-line is shown in fig- 

 ure 25. It was noted in the account of the 19 mm. embryo 

 and appears more clearly in the 32 nun. embryo, that if this angle 

 be followed backward it passes behind the attachment of the 

 hemisphere to the thalamic wall, and then as a diagonal groove 

 downward and forward across the lateral wall of the hypothala- 

 mus and ends at the optic chiasm. This is the di-telencephalic 

 groove (figs. 14 and 19, d-l.gr.) of Johnston. 



With the preoptic recess and the velum transversum fixed, 

 the extent of the telencephalic roof plate is determined. Be- 

 tween the preoptic recess and the velum transversum should 

 appear the lamina terminalis, recessus neuroporicus, lamina 

 supraneuroporica, recessus superior, tela chorioidea telencephali 

 medii, and paraphysis. 



c. Paraphysis. Just in front of the velum in each embryo 

 is a small arch (figs. IS, 20 and 25, p.a.) relatively largest in 

 the 19 mm. embryo, and smallest in the 32 nun. embryo. In 

 each case it lies immediately in front of the velum transversum. 

 In each case also the lateral choroid plexuses arise from its 

 sides. There can be no doubt that this is the paraphysal arch. 

 No indication of the development of a glandular structure 

 could be found. The resemblance of the paraphysal arch in 



