Charles Sedgwick Miuot 91 



velum transversum — compare below, Fig. 12. The superior commis- 

 sure, s. c, is now in the area of its cross section fully equal to the 

 posterior commissure, p. c, and must be regarded as of great morpho- 

 logical importance. The nervous portion of the wall of the fore-brain 

 shown in the figure in front of the paraphysal arch is very thick. 



The ependymal covering of the velum early develops a superficial 

 coat, which appears lightly colored in stained sections. In embryos of 

 22 mm. (stage of Fig. 5) this border is easily seen on the anterior sur- 

 face of the velum, but not on the posterior surface. In embryos of 

 28 mm. the border is conspicuous, but is thinnest at the base of the 

 velum, and gradually thickens toward the inferior edge. The nature 

 of the border is uncertain; the appearance is probably not due to cilia, 

 but suggests rather the formation of secretory spherules, such as I have 

 recently discovered in the cervical glands of the human uterus, the 

 Wolffian body of the pig, and the kidney of the frog. These spherules 

 are formed from the free ends of the cells, and resemble those which, 

 as Mingazzini has shown, are formed on the inner ends of the absorbing 

 epithelial cells of the mammalian intestine. I wish, however, not so 

 much to offer a definite interpretation of the border, as to suggest a 

 line of investigation. 



In order to render clearer the relations of the velum Figs. 11 and 12 

 have been added. These are both from a series of transverse sections 

 of an embryo of 28 mm., or of the same length as the embryo from 

 which Fig. 6 is taken. The plane of Fig. 11 is approximately that of 

 the dotted line A in Fig. 6. It passes, therefore, through the mid-brain, 

 M. B., and fore-brain, F. B. Next the former is the oval section of 

 the stalk of the epiphysis, Ep., crowded into the space between the two 

 commissures. The posterior commissure, p. c, apparently belongs 

 wholly to the mid-brain, as can be seen in the sagittal sections also. 

 The development in Kecturus, in the chick, and in the rabbit and pig 

 also indicates that the posterior commissure belongs to the mesen- 

 cephalon. When it is further remembered that its fibres in man are of 

 mesencephalic origin, we must conclude that the traditional description 

 of the commissure as situated in the roof of the third ventricle is erro- 

 neous, since the commissure does not belong to the diencephalon. Ke- 

 turning to Fig. 11, the superior commissure, s. c, runs in the cerebral 

 tissue of the post-velar arch — compare Fig. 6 — between it and the 

 velum proper, V, appears the narrow space of that arch; laterally this 

 space curves towards the mid-brain. The velum, V, stretches from 

 side to side of the brain, it consists of a thin central layer of mesen- 

 ch-\Tna continuous laterallv with the mesenchyma surrounding the 



