736 PROFESSOR W. C. M'INTOSH AND MR E. E. PRINCE ON 



of the enteron formed by an arch of columnar hypoblast, hy, and a floor of nucleated 

 (periblastic) protoplasm, 'per, the ill-defined ascending interspace or canal, nee, being 

 bordered by indifferent cells, and opening by means of the blastopore into the dorsal 

 groove above. This dorsal groove is more fully treated of on another page, and it can 

 be no other structure than the primitive involution forming the medullary canal in so 

 many forms, but in Teleosteans simply appearing as a transient, ancestral reminiscence, 

 and, except for this, now obliterated. Certainly its connection with the subsequent 

 permanent neural cavity cannot be demonstrated. 



So rapidly does the dorsal groove become effaced that in a large series of sections of 

 early stages none indicate this structure favourably; but a reference to Oellacher's well- 

 known figures (No. 114) sufficiently shows this, the deep groove in fig. iv. 3, Taf. ii., 

 being merely indicated in fig. vii. 5, Taf. iii.; while the figs, in Taf. iv., such as fig. iv. 1, 

 show no trace of it, nor can the permanent cavity be said to be more than foreshadowed. 

 Owing to the rapid and complete obliteration of the medullary groove, the absence of a 

 post-anal canal has been generally accepted for Teleosteans, and for this reason Balfour, 

 though adding a query to his cautious statement, concluded that no neurenteric passage 

 was " apparently developed" (No. 10, p. 286). Balfour and Parker (Phil. Trans., 1885, 

 ii. p. 365) speak of the neural canal arising in Lepidosteus as a slit-like lumen, and not due, 

 as supposed by Oellacher for Teleostei, to an actual absorption of cells. " When first 

 formed, it is a very imperfectly defined cavity, and a few cells may be seen passing right 

 across from one side of it to the other " (fifth day after impregnation). The connection in 

 Teleosteans between the primitive enteron, no other than the gastrula-cavity (see page 

 713), and the primitive dorsal groove cannot be questioned if our interpretation of figs. 

 9, 21, and 22, PI. III., be correct, for the continuity of this groove, nee, and the blastopore, 

 bp, is very apparent. The formation of a neural canal by a dehiscence of neurochordal 

 cells is a secondary process, and the Teleostei therefore form no exception to the condition 

 which so widely obtains in other Vertebrata, and which was demonstrated by Gasser in 

 birds, by Kowalewsky, Balfour, His, and others in Elasmobranehs, by Owsjannikow in 

 Oyclostomes, and by Gotte and others in Amphibians. 



Medullary Groove. — The permanent neural canal is formed comparatively late in 

 osseous fishes, whereas in most vertebrates its appearance as a groove on the dorsum is a 

 very early feature in development. For a short period, soon after the optic vesicles are 

 defined, a transient longitudinal indentation passes along the median dorsal line from 

 the head to the tail, just as Lereboullet figures (No. 95, pi. ii. fig. 36). It may be 

 regarded as actually reaching to the lip of the blastopore, though the depression is 

 so slight, in the extreme posterior region, that it is in some cases indistinguishable. 

 In Rana at a certain stage the hind part of the neural groove cannot be made out. 

 Spencer, however (No. 151, p. 97), found that it extends quite to the caudal margin, 

 but in this latter region it is obliterated — the cavity closes up, and the nervous cord 

 becomes solid. The hind end of the trunk in the embryonic Teleostean often appears 

 like a flattened plate, in which the neurochord spreads out like a spatula (PI. III. fig. 16). 



