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Part III. — Thirteenth Annual Report 



by a proliferation of cells from the thin roof, and are developed ventrally 

 and laterally from the dorsal sulcus. At this point the external covering 

 of the brain, which is for the most part a thin layer of tissue, is greatly 

 thickened, and the tegmen of the cranium, composed of loose cells, fills 

 up the deepened and broad sulcus. Along with the corresponding eleva- 

 tions in the floor of the mid-brain, they form constrictions of the lumen, 

 marking off the two lateral limbs of the optic ventricle from the median 

 portion. Here the ventricle becomes H-shaped, the ventral prolonga- 

 tions of either limb being directed slightly outwards. 



In post4arval forms, the tectum increases greatly in thickness, and the 

 resulting predominance of the mid-brain is never lost. Owing to this 

 great increase, which takes place from above downwards, the capacity of 

 the space forming the optic ventricle is greatly lessened (Plate XII., fig. 5, 

 et eeq. f v.o.). 



In sand-eels of about 5 mm. in length, the fibrous tract of the tectum, 

 as seen in longitudinal section, is a rounded cap (Plate XII., figs. 6, 7, 8), 

 and only covers about half the optic lobes along their anterio-posterior 

 axis. The anterior and posterior extremities are composed of cellular 

 matter as in larval stages, but the vesicular area has greatly increased 

 with the advance in growth. The anterior boundary is subtended by the 

 pineal body lying in front, only a deep cleft separating them. This deep 

 cleft is immediately above the fibrous tract or labium invaginatum 

 noticed by Holt*" (figs. 19 and 32, t.f.), and it is seen below the anterior 

 ventral comu of Plate XII. , fig. 8. The posterior part of the optic lobe 

 has a thin roof, where it is about to bend downwards and forwards to 

 form the fornix or valvula cerebelli (PI. XII., fig. 9, v.e.) The fornix 

 bounds the optic ventricle posteriorly, and it has attained a great develop- 

 ment both downwards and forwards since it was noticed in larvae ten days 

 old (PI. XI., fig. 11, v.e.). In sand-eel embryos, before and after hatching, 

 the fornix is the anterior border of the transverse furrow noticed on the 

 dorsal aspect dividing off the mid from the hind-brain, and the deepening 

 of this furrow underneath the tectum lobi optici lengthens the fornix as 

 well as the cerebellum. 



Where the cellular portion extends over the whole of the tectum as is 

 seen in the slightly oblique vertical longitudinal section (PI. XIL, fig. 9, 

 t.l.o.), the cells in the roof are packed closer together than on the ventral 

 side of the tectum, where they are fairly uniform, but more loosely 

 arranged. A reference to PI. XIII., figs. 6, 7, t.l.o., which are sections of 

 a 12 mm. post-larval sand-eel, shows how it is possible to obtain a 

 longitudinal vertical section through the tectum, which exhibits no patch 

 whatever of fibrous matter in the roof. The sections taken on either side 

 Of PL XII. Ag. 9, in the same post-larval form, viz. PI. XII., figs. 5 and 6 

 on one side, and figs. 7 and 8 on the other, have fibrous elements in the 

 roof of the tectum. 



In 18 mm. sand-eels the tectum has greatly extended laterally and 

 ventrally on both sides, till it partially covers the inferior lobes (PI. XIII. 

 figs. 6 and 7, tl.o.), being separated in fig. 6 from the lobi inferiores (Li.) 

 by the oculo-motor nerve (III.). The fibrous tract in the tectum is not 

 united to its fellow of the opposite side, but a double median cap of cells 

 intervenes, and the tissue abutting on the oculo-motor nerve is also 

 cellular. Scattered nerve cells are found in the fibrous tracts. Further 

 back (PI. XIII., fig. 7) the tectum (t.l.o.) is almost entirely cellular, the 

 only fibrous matter being i shallow tract on the outside ; and the tectum 

 itself contains a Y-shaped prolongation of the optic ventricle (v.o.) be- 

 tween its halves. At its posterior border, the tectum is composed of two 

 <cellulnr prolongations which cover the adjoining portion of the hind-brain 



* Holt. Ut supra. 



