1897] MICROSCOPICAL JOURNAL. 309 



sonant with great variation in different types, but this 

 study tends to corroborate the opinion now gaining 

 ground, that this commissure in the lower vertebrates is 

 not a callosum. 



5. That part of the cerebrum so prominent in the. 

 adult, the caudatum, or elevated portion of the striatum, 

 is only found as a rather inconspicuous object in the old- 

 est embryo, but the precommissure, in which fibers from 

 the upper parts of the striatum cross, arises as the cara- 

 pace begins to form. 



6. In the roof of the brain the postcommissure is a 

 well-formed landmark in the earliest of the embryos, 

 while the commissure, bounding the opening of the epi- 

 physis, the supracommissure, shows as a mere trace in 

 the youngest embryo and attains a disproportionate 

 development in the oldest. A similar culmination in 

 growth is seen in the oldest embryo in the associated epi- 

 physis, habenae and the fiber tract extending from this 

 region to the cerebrum, a fact apparently indicating 

 that in ancestors of this group having comparatively sim- 

 ple brains these parts were of more importance, for in 

 the adult turtle they are overshadowed by the later 

 developing parts. 



7. The membranous roof in all embryos is a simple 

 unfolded membrane, clearly continuous with the para- 

 plexuses of the cerebrum. The latter, in the early 

 stages, are simple membranes, which show folds only 

 when the carapace begins to develop, and become quite 

 complex in the oldest embryo. The paraphysis, at the 

 point of union of the diaplexus with the paraplexuses, is 

 a widely open tube in all the stages, and becomes early 

 convoluted. 



8. The medicommissure, a feature which is found in 

 mammals and reptiles, but not in birds, arises in this 

 turtle only in the oldest embryo, in this being like mam- 

 mals, in which it also appears late, and showing that 



