Charles Sedgwick Minot 83 



thesC;, the first, V, is the anlage of the velum; the second, which marks 

 the boundary between the two arches, is the site for the future com- 

 missura superior; the third, which forms the posterior boundary of the 

 epiphysal anlage is the site for the future commissura posterior — com- 

 pare Fig. 5. The curve in front of the velar anlage I propose to name 

 the paraphysal arch, on account of its subsequent differentiation; the 

 curve between the velum and the epiphysal anlage may be termed the 

 post-velar arch. We thus distinguish six fundamental morphological 

 divisions in the median line of the diencephalic roof : 



1. Paraphysal arch. 



3. Velum transversum. 



3. Post-velar arch. 



4. Superior commissure. 



5. Epiphysis. 



6. Posterior commissure. 



H.B. 



Fig. 3. Embryo of 11.5 mm. Sagittal series, No. 308, section 93. x 30 diams. 



The homologues of all these parts exist probably in all vertebrates. 



In the next stage, which I have, an embryo of 13.0 mm.. Fig. 2, the 

 brain is larger, and the outline of the fore-brain has been considerably 

 modified, chiefly owing to the growth of the infundibular region. The 

 " anterior " cavities in this embryo (No. 224, sections 99-100) seem to 

 have no longer any connection with the praemandibular cavities, but 

 lie further laterad. The relations are well demonstrated by transverse 

 sections — thus No. 223, section 99, shows both the " anterior '' cavity 

 and the median connection of the praemandibular cavities. In both 



