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Part III. — Thirteenth Annual Report 



tectum lobi optici is increased, and it arches over the base of the hind 

 brain just as the tectum lobi optici does over its corresponding base, at 

 the point of origin of the third pair of nerves (Plate XIII., fig. 6, in.). 

 When it has attained this great breadth, the fourth ventricle is extended 

 laterally nearly across the whole brain (Plate XI II., fig. 8, v. 4). These 

 lateral extensions, represent the expanded lateral arms of the dagger- 

 shaped aqueduct of Sylvius, seen in the preceding figure (Plate XIII., 

 fig. 7, a.q.), as succeeding sections (Plate XIII., fig. 9), are examined, the 

 lateral prolongations of the fourth ventricle (v. 4), diminish in size, and 

 the deep furrow separating the two halves of the base of the brain, as 

 seen in a previous section, becomes also less. This contraction in the 

 fourth ventricle is due to the narrowing of the hind brain. The cere- 

 bellum at this point consists of two lobes, separated from one another by 

 a Y-shaped upward prolongation of the cavity of the ventricle, and from 

 the basal portion of the hind brain, by the transverse arms of the fourth 

 ventricle. The ventral prolongation which was so marked in the 

 aqueduct of Sylvius, and in the fourth ventricle anteriorly, has now almost 

 disappeared, and there is in the posterior part of the hind brain only au 

 indication of partial separation into two longitudinal halves. The 

 cerebellum in post larval-forms of this size ends in a small tongue-shaped 

 mass, not unlike what is found in adults. 



Behind the cerebellum the fourth ventricle is covered by a thin layer 

 of tissue in close union with the pia mater (PI. XIII., fig. 10, t.v. 4). In 

 post larval sand-eels, the tectum of the fourth ventricle has become much 

 less in size. This is chiefly due to the posterior elongation of the cere- 

 bellum. In larval fishes (PI. XL, figs. 7-9, 11, t.v. 4), the tectum covers the 

 whole of the base of the hind brain from side to side, and from the posterior 

 margin of the tectum lobi optici to the junction of the medulla oblongata 

 with the spinal chord, but when post larval stages are reached, its trans- 

 verse as well as its longitudinal dimensions do not increase with the 

 increase of the brain. In larval sand-eels it is only a thin layer of tissue, 

 but in post-larval forms it has attained to the tela vasculosa condition 

 exhibited in the adult, although it is not further complicated by foldings 

 found in other vertebrates. 



The Base of the Brain. — The medulla oblongata forms the floor of 

 the fourth ventricle, and gradually merges posteriorly into the spinal 

 chord. The open fourth ventricle, which is arched over by the cerebellum 

 and the thin layer, its tectum, is continued into the central canal of the 

 spinal chord. The forward boundary of the medulla is not marked off 

 from the floor of the brain in front. Anteriorly it is a flattened cushion 

 with a furrow on its dorsal surface dividing it into two longitudinal 

 halves. This apparent flattening is gradually lost posteriorly till in 

 section it has the appearance of a more or less round rod which passes 

 into the rounded spinal chord. 



In larvae of ten days (PI. XL, figs. 7-11, m.o.), the dorsal longi- 

 tudinal furrow, which is the prolonged ventral limb of the aqueduct of 

 Sylvius as already mentioned, cleaves it nearly into two halves, the 

 sulcus being broader at its open end than at the bottom (fig. 7, 8.) As 

 we proceed posteriorly (fig. 9) the two lateral halves of the medulla are 

 brought close to one another, and the furrow is deep but narrow. The 

 furrow shallows behind, but at its lower end it is continued into the 

 canalis centralis of the neurochord (fig. 9, sp.c). The distribution of the 

 vesicular and fibrous matter remaius fairly constant. The base of the 

 floor of the brain from below the widened optic ventricle is composed 

 chiefly of two areas of white fibrous substance with nerve cells inter- 

 spersed in it. In the region near the mid-brain, the base is flattened 

 against the basal parts of the anterior portion of the brain which come to 



