DEVELOPMENT AND LIFE-HISTORIES 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, Pl. UI. fig. 4), quite a deep fissure, showing itself 
earliest anteriorly, and extending, as VAN BampBeke describes, in the form of “a 
slight depression,” the “sillon primitif” (see his fig. 12, pl 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 Hsow, 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. 
Kurrrer maintains that it is not by any means the homologue of the medullary 
groove of higher Vertebrates (No. 87, p. 251); while OkLLacnER 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. Scorr (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.{ 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 embryo 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. Ryper makes a passing reference to 
this (No. 141, p. 564). 
+ This epidermic involution in Petromyzon has now been disproved by the recent investigations of SairLey 
(No. 150, p. 9). 
t Compare the observations of Batrour and Dercuton on the chick (No. 19, p. 183). 
VOL. XXXV. PART III. (NO. 19). 6B 
