10 SUMMARY OF CURRENT RESEARCHES RELATING TO 



a single row at its most anterior extremity, invested by a layer of 

 smaller cells which are richer in protoplasm, the whole contained 

 within a structureless envelope (primitive or chordal sheath of Gegen- 

 baur). Beyond this cuticle we encounter successively (a) Gegeubaur's 

 skeletogenous sheath, (h) its limitaus externa, (c) the outlying basilar 

 cartilage, and ((/) the perichondrium. The changes in shape and size 

 which the notochord displays as we proceed from before backwards 

 are of much interest, but could not be profitably explained without 

 the figures. 



Dr. Kabl-Riickhard has not traced the actual origin of the hypo- 

 physis, but a comparison of his sections confirms the accui-acy of 

 Balfoiu''s observations. The early development of this structure in 

 Selachians agrees with its beginning in birds, as since seen by 

 Mihalkowicz and others. The hypophysis may be assumed to 

 occupy the same position in all the Craniata. 



As to the middle trabecvila of Eathke, neither can this name nor 

 that subsequently proposed for the same part by Eeichert be con- 

 sidered appropriate. Posteriorly, it passes continuously into the 

 connective tissue occupying the floor of the vertebral canal and 

 insinuating itself between the surface of the medulla and the cartila- 

 ginous investment of the notochord, to constitute at a later stage the 

 membranes of the great nervous centres. The connection between 

 these membranes and the middle trabecula demonstrates that the 

 latter is at first nothing more than a specially developed vascular 

 extension of the pia mater. 



II. It is manifestly important to fix the data for exact comparison 

 of the brains of the higher vertebrates with those of the lower. It 

 would be an advantage if these data were beyond the reach of con- 

 troversy, since young anatomists are puzzled when they find that so 

 high an authority as Gegenbaur has wavered between the interpreta- 

 tion of the fish's brain promulgated by Miklucho-Maclay and the 

 more orthodox (and usual) view which has found its chief modern 

 supporter in Stieda. More recently, the copious and original work of 

 Fritsch, while containing valuable additions of detail, has further 

 involved the whole subject in much perplexity. 



The pineal gland, although functionally of no interest, here 

 afi'ords a valuable landmark. By ascertaining its true position, we 

 advance towards the solution of the problem now before us. In all 

 the higher vertebrates, as Stieda insists, it lies dorsally between 

 structures which correspond to the primary first and second cerebral 

 vesicles. But Fritsch asserts that such a position must be ascribed 

 to secondary displacement, and supports this opinion by an appeal to 

 horizontal sections of the brains of the eel and frog. In accordance 

 with these, he maintains that the parts immediately behind the pineal 

 gland in fishes and batrachians belong, not to the middle, but to 

 derivatives from the first primary cerebral vesicle. He thus errs 

 both as to logic and matters of fact. For if the pineal gland really 

 shift in this way, its utility to the morphologist becomes questionable. 

 However, Balfour and Ehlers have demonstrated that in elasmo- 

 branch fishes the pineal gland is developed just as among the higher 



