DEVELOPMENT AND LIFE-HISTOKIES OF TELEOSTEAN FISHES. 737 



This flattened condition frequently continues for some time after the closure of the 

 blastopore (PL III. figs. 18 and 20). It is merely a shallow groove, barely perceptible 

 posteriorly, and does not therefore enclose the blastopore, which remains open for a 

 short time, as a pore with a corrugated margin, but in the cephalic region the groove 

 forms, as in the gurnard (dg, PI. III. fig. 4), quite a deep fissure, showing itself 

 earliest anteriorly, and extending, as Van Bambeke describes, in the form of " a 

 slight depression," the " sillon primitif" (see his fig. 12, pi. ii.), to the tail. In the 

 forms here considered, the two lateral folds are by no means sharply ridged, and 

 viewed from above the furrow is difficult to make out ; and is thus unlike the condi- 

 tion in Esox, which Lereboullet says is distinctly marked by two parallel lines 

 — the groove being deepest in the mid-trunk, and gradually disappearing before and 

 behind (No. 95, p. 516). In the mid-trunk, he remarks, it likewise remains open 

 for the longest time (p. 528). This groove is, however, as before suggested, merely 

 a reminiscence of the ancestral condition, and wholly disappears chiefly by the horizontal 

 widening out of the embryonic trunk as the blastoderm proceeds to envelop a larger extent 

 of the vitelline globe.* This is evidently the case posteriorly, but in the head-region 

 obliteration is achieved less by elevation of the base of the groove than by coalescence of 

 its walls. 



Kupffer maintains that it is not by any means the homologue of the medullary 

 groove of higher Vertebrates (No. 87, p. 251); while Oellacher regards it as pro- 

 duced by the formation of the carina, the furrow deepening as the keel presses down- 

 ward, and it is certainly true that the furrow is produced subsequent to the growth 

 of the carina, and does not, as he proved, become the medullary canal ; but the view 

 adopted in these pages, that the carina is a neurodermal proliferation and the dorsal 

 furrow an ancestral reminiscence, agrees best with appearances in life and in sections. 

 Certainly no confirmation is given to Calberla's opinion that ectodermal cells are 

 involuted along the central dorsal line to form the epithelial lining of the neural canal, 

 as the same authority, supported by W. B. Scott (No. 145), holds to be true for 

 Petromyzon.t 



As a matter of fact, the dorsal groove in Teleosteans does not appear to become any 

 organ, but wholly passes away. It is subject to great variation, just as in the chick, for 

 at times it is apparently entirely wanting, or at most is represented merely by a 

 shallow depression, which may be discernible in the short posterior part of the indifferent 

 caudal mass.J This posterior mass of indifferent cells, to which reference has frequently 

 been made, forms the termination of the embryo (PL III. figs. 18, 20-22), where it 

 reaches the lip of the blastopore, bp. In it neither neurochord, notochord, nor mesoblastic 



* The superficial extent of the Teleostean emhryo is a characteristic feature, and the dorsal groove is thus opened 

 out on account of the large bulk of the yolk upon which the germ lies flattened. Ryder makes a passing reference to 

 this (No. 141, p. 564). 



t This epidermic involution in Petromyzon has now been disproved by the recent investigations of Shipley 

 (No. 150, p. 9). 



% Compare the observations of Balfour and Deighton on the chick (No. 19, p. 183). 



VOL. XXXV. PART III. (NO. 19). 6 B 



