146 



MOLLUSCA 



that in the chick the orifice of closure of the overspreading 

 blastoderm does not represent the whole of the blastopore, 



(1) 



(7) 



FIG. 121. Development of Loligo. (1) View of the cleavage of the egg during 

 the first formation of embryonic cells. (2) Lateral view of the egg at a little 



ce protrusion of the median portion of tl 



the yelk or food-material lying within it. (Original.) 



but that this is in part to be sought in the widely-separated 

 primitive streak. The present writer has little doubt that 



a structure corresponding to the primitive streak of the 

 chick, and lying near the klastic pole, will be found in 

 Sepia and Loligo, and the strange vesicular origin of the 

 mid-gut will be traced to and explained by it. 



Leaving this difficult question of the cell-layers of the 

 embryo, we would draw the reader's attention to the series 

 of sketches representing the semi-transparent embryo of 

 Loligo, drawn in fig. 121. When the cleavage cells have 

 nearly enclosed the yelk, the upper or embryonic area 

 shows the rudiments of the centro-dorsal mantle-sac or 

 pen-sac, the mouth, the paired optic pits, and the paired 



Fir:. 122. Section through the still open shell-sac occupying the centro-rlorsal 

 area of an embryo of Loligo ; the position is inverted as compared with fig. 

 }'2l (3) and (7). fp, outer cell-layer; m, middle cell-layer; in', deep cell- 

 layer of fusiform cells ; i/, the granular yelk or food-material of the egg ; 

 shs, the still open shell-sac. (From Laukester.) 



otic pits (fig. 121, (3), (5)). The eye-pits close up (fig. 

 119), the orifice of the mantle-sac narrows, and its margin 

 becomes raised and freely produced as mantle-skirt ; 

 at the same time an hour-glass-like pinching in of the 

 whole embryo commences, separating the embryo proper 

 from the so-called yelk-sac (fig. 121, (4) ). Around the 

 " waist " of constriction, pair by pair, ten lobes arise (fig. 

 121, (8)), the arms of the fore-foot. It now becomes 

 obvious that the yelk-sac is but the median surface of the 

 fore-foot bulged out inordinately by food-yelk, just as the 

 hind region of the foot is in the embryo slug (see fig. 72**, 

 and explanation). Just as in the slug, this dilated yelk- 

 holding foot is rhythmically contractile, and pulsates 

 steadily over the area g in fig. 121, (4). At this stage, 

 and long subsequently, the mouth of the young C'ephalopod 

 is in no way surrounded by the fore-foot, but lies well 

 above its nascent lobes (e in fig. 121). Subsequently it 

 sinks, as it were, between the right and left most anterior 

 pair of the series, which grow towards one another and 

 fuse above it, and leave no trace of their original position 

 and relations. Fig. 121, (6) gives a view of the postero- 

 dorsal surface of an embryo, in which the important fact 

 is seen of the formation of the funnel or siphon by the 

 union of two pieces ('/), which grow 1 up each independently, 

 one right and one left, like the sides of the siphon of 

 Nautilus or the swimming lobes of a Pteropod, and subse- 

 quently come together, as shown in (9), where the same 

 letter q indicates the same part. The explanations of figs. 

 121 and 123 are given very full, and here, therefore, we 

 shall only allude to two additional points. A curious mass 

 of tissue of unknown significance occurs in the orbit of 

 Bibranehs, known as the white body (w in fig. 120). A 

 strongly-marked invagination just above the orbit is a very 

 prominent feature in the embryo of Loligo, Sepia, and 

 Octopus, and appears to give rise to this so-called white 

 body. This invaginated portion of the outer cell-layer is 

 seen in fig. 121, (8) and (9), lettered c ; in fig. 123, A and 

 B, it is lettered wb. 



Lastly, in fig. 123, A, the origin of the optic nerve- 

 ganglion ny from the cells of the middle layer should be 

 especially noticed. In some other Molluscs the nerve- 

 ganglia have been definitely traced to the outer cell-layer, 



