86 On the Morphology of the Pineal Eegion 



ness; accordingly the outer surface of the mid-brain and of the fore- 

 brain, F. B., also the summit of the epiphysis, Ejp., are all nearly as 

 close to the outer surface of the head as before, while on the contrary 

 the orifice of the epiphysis, the posterior commissure and the superior 

 and the edge of the velum are all much more remote from the surface 

 than before. Comparison of the successive stages shown in Figs. 4-10 

 will suffice to demonstrate that increasing downward extension is the 

 method of development by which all these parts elongate. This com- 

 parison will also demonstrate that the summit of the epiphysis remains 

 quite close to the ectoderm, while the continued development of the 

 mesoderm is increasing the distance between the epidermis and the 

 mid-brain and fore-brain both, so that the epiphysis comes to project 

 more and more above the level of the brain. Two further character- 

 istic features are illustrated by Fig. 5, first the small growth of the 

 post-velar arch, so that the distance between the base of the velum and 

 the orifice of the epiphysis is barely more than sufficient for the su- 

 perior commissure. It is probable that the small size of the post-velar 

 arch is a special characteristic of the elasmobranch type. In other 

 vertebrates the post-velar arch has considerable extension in the sagittal 

 direction, as in Accipenser according to Kupffer, and in ISFecturus, Fig. 

 13, and Gallus, Fig. 14. Second, the ependyma on the posterior side of 

 the velum has grown thinner, except that towards the inferior edge 

 of the velum it thickens and is then reflected over the edge onto the 

 anterior surface. Finally it must be noted that up to this stage the 

 velum has elongated more rapidly than the epiphysis, after this stage 

 on the contrary the epiphysis elongates much more than the velum. 



In embryos of 28.0 mm. the permanent relations are already clearly 

 indicated. The most important advance has been the thickening of the 

 wall of the fore-brain, except as seen in the sagittal sections towards 

 the velum, where the wall corresponding to the area of the paraphysal 

 arch remains thin. The differentiation of this arch therefore occurs 

 quite late, and may perhaps be best described as resulting from an 

 arrest of the histological development, which just in front of the arch 

 progresses rapidly, there causing the brain wall to thicken, and to 

 change into nervous tissue proper. Owing to the continued down- 

 growth of the parts a deep fold or cleft is formed between the mid- 

 brain, M. B., and fore-brain, F. B. In and near the median plane the 

 space of the cleft is almost filled by the epiphysis, as shown in the 

 figure — the wall of the epiphysis being almost in contact with the wall 

 of the mesencephalon behind, and with the paraphysal arch in front. 

 Laterally the cleft between the two vesicles is filled only with mesen- 



