No. 2.] DEVELOPMENT OF PETROMYZOISr. 273 



folds into the cavity, thus forming the choroid plexus of the mid- 

 brain. This is not shown in any of the figures, but does not 

 differ except as to size from the plexus of the hind-brain (Figs. 

 13 and 14, PI. VIII). Special protuberances to form the optic 

 lobes do not appear till late in larval life ; obviously a retarda- 

 tion, due to the rudimentary condition of the eye during the 

 larval state, which, we have already seen, has a profound effect 

 upon the development of the brain. 



{c) The Hind-Brain. — This region of the encephalon has 

 likewise a very simple course of development. It is from the 

 beginning very much larger than either of the anterior regions 

 of the brain, though proportionally short when compared with 

 the length of the entire animal. In early stages it but slightly 

 exceeds in diameter the spinal cord, but at the time of hatch- 

 ing it has considerably increased in size. The ventricle is nar- 

 row, but somewhat expanded at the top and roofed in by a 

 single layer of cells (Fig. 29, PI. X). The white matter is con- 

 fined to bands on the sides, while most of the walls consist of 

 closely-crowded ganglion-cells. The fibres increase rapidly in 

 number, and in larvae of 15-20 mm. in length make up most of 

 the substance of the hind-brain (Fig. 34, PI. X), the cells being 

 reduced to a layer surrounding the ventricle and scattered nuclei 

 for the roots of the cranial nerves. At the same time, the hind- 

 brain changes its form (compare Figs. 29 and 34, PI, X), the 

 dorsal region of the ventricle widening out, and the roof be- 

 coming very thin and membranous. A vascular network appears 

 and forms a choroid plexus which projects into the cavity of the 

 ventricle, though separated from it by the membranous roof 

 (Fig. 13, PI. IX). This thinning of the roof is widest anteriorly, 

 and tapers posteriorly to a mere slit (Fig. 15, PI. IX), There 

 appear very early in both mid and hind-brains a series of 

 alternate widenings and narrowings of the ventricles, giving the 

 internal border, when seen in horizontal section, a scalloped ap- 

 pearance, which persists for a considerable period (Figs, 15, 

 PI, IX, and 23, PI. X), In the hind-brain, some of the expan- 

 sions, at least, seem to stand in relation to the roots of the cranial 

 nerves (see Fig. 23), as Balfour has described for the vagus 

 roots in the Selachian embryo ; and they may possibly be re- 

 garded as of segmental value in the mid-brain, though this is 



