CEPHALOPODA. 



63: 



crura of the funnel, the cephalic lobes and eyes, but not in the gills. 

 The two crura of the funnel elongate ; the eyes become reniform. 

 Two other (dorsal) pairs (3, 4) of arms appear, and now the mouth {g) 

 is established as a median semilunar depression with a raised border. 

 The mantle (which is built up from below or behind) becomes larger 

 and more prominent, its free border begins to be detached along its 

 ventral side, and the germinal membrane expands upon the yolk. 

 The mouth liberates its anterior border and slides from the vitelline 

 mass. The crura of the funnel {Jig- 234, d) approximate ; the small 

 gills (ib. c) begin to get under the still-growing border of the 

 234 mantle (a), and a small pulsating 



eminence (w), between the gills 

 indicates the systemic heart. 

 The two tentacles (2) now rise 

 near the posterior cephalic lobes, 

 and, from the beginning, are 

 more slender than the ordinary 

 arms (1, 3, 4, 5). The miniature 

 Sepia is recognisably sketched out : but all its parts, save the gills 

 and funnel-lobes, are ciliary. By this ciliary action the embryo 

 Loligo rotates in its chorion ; but the yolk of the Sepia is too big, 

 and the albumen yields to the microscopic dynamic and flows over the 

 embryonal disc. The crura of the funnel first coalesce at their dorsal 

 margin : the gills, not yet quite covered by the mantle, begin to show 

 their subdivisions : the eyes (ib. b) receive their pigment : the arms 

 converge towards the centre of the cephalic lobes, but they show no 

 trace of suckers : the yolk (p) is now almost quite surrounded by the 

 germinal membrane (/). The anterior or ventral divisions of the infun- 

 dibular crura lay with their free, arched, thick borders overlapping, 

 and finally coalesce, completing the dibranchiate tubular funnel, the 

 apex of which now reaches nearly to the base of the arms. The 

 sepium, or internal shell {Jig. 235,/), the digestive canal {b\ with its 

 appendages, and the ink-bag (e), are formed suc- 

 cessively : the basis of the nervous system was 

 probably coeval with the appearance of the 

 cephalic lobes and cartilage. In the course of 

 the investment of the yolk, and the concomitant 

 growth of the embryo, the vitelline sac becomes 

 constricted into an external (c) and an internal 

 {d) portion : the constricted portion is gradually 

 elongated, and becomes a narrow^ canal, extend- 

 ing from the head by the side of the mouth. 

 The external yolk progressively becomes in- 

 ternal as the latter is absorbed, and is finally 



235 



Embryo Sepia. 



