PINEAL REGION IN MAMMALIA 99 



no need of adding a special picture. The same is true of the 

 three main subdivisions of the brain as seen in transverse sec- 

 tions (fig. 28). In figure 33, a cat embryo of 10.7 mm., there is 

 a very low paraphysal arch and only a slight fold for the velum, 

 which is less well marked than in earlier stages. Above it ap- 

 pears a wide projection in the diencephalic roof somewhat 

 similar to that seen in figure 32. The rest of the brain roof 

 shows merely the other arches. In an embryo of 24 mm. (fig. 

 34) the paraphysal arch is reduced to a mere slit, the velum is 

 also very insignificant and immediately above it there is a large 

 forward projection of the diencephalon followed by a mass of 

 plexus which involves the whole of the roof. This projection 

 has the same relative position as the one shown in human em- 

 bryos but seems to become more involved in the plexus forma- 

 tion and to lack the tubular outgrowths which are so char- 

 acteristic of the former. A small epiphysis which can be seen 

 in embryos of 15 mm. is shown here inclined backward between 

 the superior and posterior commissure. The largest embryo 

 cut sagitally is shown in figure 35. It is an embryo of 39 mm. 

 and is in the main an exaggerated picture of that seen in figure 

 34. The greatly reduced size of the paraphysal arch and velum 

 is to be noted and also the excessive plexus development which 

 involves the whole of the diencephalic roof up to the suprapineal 

 recess. The pars intercalaris occupies a very prominent space 

 in the roof of the diencephalon in this stage and in that shown 

 in figure 34 and is not as much suppressed by the pressure from 

 the midbrain roof as is usually the case. No paraphysis could 

 be found in any of the cat embryos in the collection. Owing 

 to the small size of the paraphysal arch and of the velum and 

 to the extreme development of the diencephalic plexus, it is 

 often difficult to fix accurately the true position of the velum 

 and misinterpretations are easily made. The great temptation 

 is to place the velum rather further back along the brain roof 

 and describe the fold or folds which then seem to belong to the 

 telencephalon as the paraphysis. The author feels convinced, 

 however, that the velum in the above figures is properly placed, 

 thereby confirming all the above folds to the diencephalon and 



