2i8 ORR. [Vol. I. 



Anteriorly, at the base of the fore-brain {FB), appears the 

 recurved end of the notochord {N.) in cross-section; and 

 laterally from it, extending partly around the base of the fore- 

 brain on both sides, is a double wall of cells (CZ), pressed 

 close together for a distance, but separating distally to form a 

 lumen. This lumen is enclosed by the distal union of the two 

 walls. The cavity is round, excepting a conical point at the 

 place where the two walls separate ; so that proceeding toward 

 the notochord the lumen gradually disappears, though the 

 walls remain distinct as far as to the notochord. An examina- 

 tion of the whole series shows the structure in question to be a 

 rod-like body with a distal hollow enlargement. Meeting each 

 other at the median line, the two parts enclose and fuse with the 

 anterior extremity of the notochord. The hollow enlargement 

 is the head-cavity. As I am unacquainted with -any name for 

 such a median connection of the head-cavities and notochord, 

 I shall hereafter refer to it as the coelenteric zone.^ 



In the embryo of series B, that connection of the notochord 

 and rudiment of the coelenteric zone with the epiblast has dis- 

 appeared. In the embryo of series A, the cell-mass {HC) at 

 the anterior end of the notochord was fused with the epiblastic 

 roof of the hypophysis, and not completely separated from the 

 hypoblast. In the present stage, however, the separation from 

 the hypoblast is complete. Section 21, B (six sections behind 

 20, B) , shows also that the separation of the notochord and the 

 coelenteric zone from the hypophysis is complete, for here noth- 

 ing but a thin band of the general interstitial tissue separates 

 the intestine from the fore-brain anteriorly to the coelenteric 

 zone. The second section behind this shows in tangential 

 section the epiblastic roof of the hypophysis {Hph, Fig. 22, B) 

 As in the preceding stage, it is in close contact with the base of 

 the fore-brain. Three sections farther backward. Fig. 23, B, 

 shows the oral fusion of the hypoblast and epiblast, also the 

 laterally open cavity between the oral fusion and fore-brain, 

 which becomes the cavity of the hypophysis. In Figs. 22, B, 



' Marshall (On the Head-Cavities and Associated Nerves of Elasmobranchs. 

 Quart, yourn. of Mic. Sci., Vol. XXI., N.S., 1881) has described homologous and 

 nearly similar relations as existing in the Elasmobranchs. Van Wijhe (Ueber d. 

 vorderen Neuroporus, etc., Zool. Anzeig., pp. 683, 684) supposed, for theoretical 

 reasons, that some such connection mujt exist or have existed. 



