256 CEPHALOPODA. 



(ra). In front of and somewhat laterally to these folds, a rather long 

 paired ridge appears (O, htf) running at first almost parallel with the 

 branchial folds; it then passes back round the "mantle" at the 

 margin of which it can be seen even on the opposite side of the embryo 

 (i.e., when the embryo is viewed from the oral side, B, htf). This 

 paired ridge, which appears very early, assists in the formation 

 of the funnel, the chief part of which, however, is derived from a pair 

 of folds which arises somewhat later more anteriorly (0, vtf). These 

 two pairs of ridges will be distinguished as the posterior (htf) and 

 anterior (vtf) funnel-folds. At first they are not prominent, and this 

 is still the case with the anterior pair at the stage depicted in Fig. 

 116 C, but they rise up more and more and then become very con- 

 spicuous (Fig. 141, //•, p. 297). Between the anterior ends of the two 

 posterior folds a slight curved prominence appears, apparently uniting 

 the folds of the two sides (D). A very narrow superficial prominence 

 now also connects the anterior folds ; this latter fold is the first 

 indication of the complete connection of the two anterior folds which 

 takes place in later stages (Figs. 117 and 118). 



When the anterior funnel-folds appear, two vesicular depressions 

 of the surface are seen behind them (Fig. 116 C, ot ; Fig. 141 A, 

 ]). 297) ; these are the otacysts which, when the posterior funnel-folds 

 shift forward, are found lying near them (Figs. 116 Z> and 117 A and 

 B). They lie also in the closest proximity to the large swellings 

 which carry the optic pits. These swellings, which are very large 

 even in an early embryonic period (Figs. 116 A and 115 B, au), 

 continue to increase in size, and give the Cephalopodan embryo a 

 highly characteristic appearance at the stage just described and 

 especially in the following stage (Figs. 117 and 118). Only a part 

 of these large projections is yielded by the eyes themselves which 

 first appear on them in the form of depressions (Fig. 116 B). These 

 depressions close later, a second depression then forming above the 

 first (primary) optic vesicle, and the lens being secreted inwards at 

 this point (<■/. Figs. 142 and 143, pp. 298 and 299). 



The formative processes hitherto described affect merely a limited 

 part of the egg, for the embryonic rudiment which extended earlier 

 over the larger part of the egg (Fig. 11")) has drawn back more 

 towards the animal pole. There is therefore a part entirely free from 

 the rudiments of organs, formed only of the yolk-mass and enclosed 

 by several cellular integuments (Fig. 133, p. 283). This is the 

 yolk-sac, which in later stages is much more distinct, the embryo 

 becoming marked oft' from it by a constriction (Figs. 117-119). The 



