96 JOHN WARKEN 



is a fold which runs across the brain at this point, in the position 

 of a paraphysis, but which cannot be regarded as the* real 

 structure. The velum is very small and the postvelar arch is 

 now filled by the diencephalic plexus. The epiphysis is of large 

 size and forms a long sack with rather thick walls and a wide 

 opening into the brain. Just in front of its opening appears a 

 small superior commissure and the posterior commissure fills all of 

 the pars intercalaris and extends back into the midbrain. Figure 

 27 is a median section of an embryo of 25 mm. constructed from 

 three different sections and is the largest one in the collection. 

 Immediately in front of the small velum is seen the same trans- 

 verse fold that appeared in the previous figure, which however 

 is deeper and extends entirely across the brain cavity. The 

 postvelar arch has become long and more dome-shaped and 

 contains masses of plexus. Above the superior commissure is 

 a very deep suprapineal recess folded backward against the an- 

 terior wall of the epiphysis. Both commissures are well devel- 

 oped but the posterior seems to be much crowded together by 

 the encroachment of the midbrain which has shortened up the 

 pars intercalaris. The epiphysis is of unusual size and has 

 developed backward over the pars intercalaris and the mid- 

 brain. Its cavity is at first wide but narrow^s towards the distal 

 end, which however is thick and expanded laterally. There is 

 apparently no real paraphysis in rat embryos up to 25 mm., its 

 place being occupied by a simple fold running across the full 

 width of the brain cavity in the paraphysal arch. The size 

 and shape of the epiphysis are also very striking. 



B. Rabbit. Early rabbit embryos give a picture of the pri- 

 mary arches essentially the same as that shown above for rat 

 embryos. Figure 28 is a transverse section of an embryo of 

 6 mm. sho^\ing the primary subdivisions of the forebrain into a 

 telencephalic segment, T, and two diencephalic segments, I.D., 

 and I ID., as described in the -sheep and pig embryos earlier in 

 this paper and in a previous article (Warren (24), figs. 34 to 37). 



An embryo of 14.5 mm. (fig. 29) has a much reduced para- 

 physal arch. The velum is rounded and as yet the postvelar 

 arch is without any plexus formation. The epiphysis is much 



