146 



MOLLUSC A 



that in the chick the orifice of closure of the overspreading 

 blastoderm does not represent the whole of the blastopore, 



T> 



if 



(1) 



(7) 



Fro. 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 

 later stage, a, limit to which the layer of cleavage-cells has spread over the 

 egg ; ft, portion of the egg (shaded) as yet uncovered by cleavage-cells ; ap, the 

 autoplasts ; kp, cleavage-pole where first cells were formed. (3) Later stage, 

 the limit a now extended so as to leave but little of the egg-surface (6) unen- 

 closed. The eyes (d\ mouth (e), and mantle-sac (?() have appeared. (4) Later 

 stage, anterior surface, the embryo is becoming nipped off from the yelk 

 sac ((/). (5) View of an embryo similar to (3) from the cleavage-pole or 

 centro-dorsal area. (6) Later stage, posterior surface. (7) Section in a 

 median dorso-ventral and antero-posterior plane of an embryo of the same 

 age as (4). (8) View of the anterior face of an older embryo. (9) View of the 

 posterior face of an embryo of the same age as (8). Letters in (3) to (9) : a, 

 lateral fins of the mantle ; 6, mantle-skirt ; c, supra-ocular invagination to 

 form the " white body " ; d, the eye ; e, the mouth ; /, 2, 3, 4 t s : the five paired 

 processes of the fore-foot ; g t rhythmically contractile area of the yelk-sac, 

 which is itself a hernia-like protrusion of the median portion of the fore-foot 

 (see fig. 72**) ; A, dotted line showing internal area occupied by yelk (food- 

 material of the egg) ; k, first rudiment of the mid-foot (paired ridges which 

 unite to form the siphon or funnel) ; J, sac of the radula or lingual ribbon ; 

 m, stomach ; n, rudiments of the gills (paired ctenidia); o, the otocysts, a 

 pair of invaginations of the surface of the mid-foot ; p, the optic ganglion ; 

 q, the distal portion of the ridges which form the siphon or mid-foot, k being 

 the basal portion of the same structure ; r, the vesicle-like rudiment of the in- 

 testine formed independently of the parts connected with the mouth, s, k, m, 

 and without invagination ; s, rudiment of the salivary glands ; ( in (7), the 

 shell-sac at an earlier stage open (see lig. 122), now closed up ; u, the open 

 shell-sac formed by an uprising ring-like growth of the centro-dorsal area ; 

 w in (5), the mantle-skirt commencing to be raised up around the area of the 

 shell-sac. In (7) me.s points to the middle cell-layer of the embryo, ep to the 

 outer layer, and h to the deep layer of fusiform cells which separates every- 

 where the embryo from 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-dorsal 

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

 121 (3) and (7). ep, outer cell-layer; m, middle cell-layer; m', deep cell- 

 layer of fusiform culls ; y, the granular yelk or food-material of the egg ; 

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



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 Cephalopod 

 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 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 

 Dibranchs, 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 ng 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, 



