DEVELOPMENT OK THK EMBRYONIC RUDIMENT — l,<>LH.(>. 



261 



as an unpaired fold (Brooks). 



.-fl 



120 .1 and />', hin). They continue to increase in breadth and 

 represenl the nuchal muscles (musculi collares) which, together with 

 the retractors (Fig. 120 .1, rt) that run backward direct to the 

 mantle, form a kind of lateral chambers, not communicating with 

 the actual funnel, i.e., with the middle funnel-cavity. The fv/txnel- 

 valve appears in the anterior part of the funnel, i.e., in the wall 

 which is in contact with the body, 



While the changes just described 

 have taken place in the funnel, 

 the mantle also has undergone 

 repeated modifications of form. It 

 now rests like a cap on the end of 

 the body ( Fig. I is, ma) as its 

 edge has extended forward further 

 and has become raised from the 

 body. The further growth of this 

 overhanging margin leads to the 

 formation of the mantle-cavity, 

 into which the gills are drawn, these 

 not having essentially, changed 

 their shape (Fig. 118, /•). The 

 increase in size of the mantle is now 

 the chief feature of its develop- 

 ment (Figs. 11S-121). The tins 

 at its upper end also increase in 

 size. The optic swellings have 

 become very la rue. It has already 

 been pointed out that in this way 

 the embryo acquires a specially 

 characteristic appearance which it 

 retains in later stages also (Figs. 

 I I'll J and IS). 



We have so far mentioned only tin' first stage in the rise >>' the 

 nrms (p. 254). From the circular fold which runs round the whole 

 embryo at the boundary line between the embryonic rudiment and 

 the yolk-sac (Fig. 1 1 (i B, ar), the separate prominences which repre- 

 sent the arms become differentiated, each first appearing as a long 

 .-welling which soon assumes a button-like form (Fig. 1 16 B-D). The 

 first pairs of arms to become distinct are the two that lie nearest 

 the funnel, the second pair of arms, however, in Lolujo attains a far 

 higher degree of development than the first and than the one that 



Kn,. 119.— Older embryo of Loluju 

 mdgaris, seen from tlie posterior 

 or funnel-side (original). /,; tunnel. 

 The rest of the lettering as in Fig. 

 U.S. The gills lie p.utly hidden 

 by the overhanging margin of the 

 mantle; between them is the anal 

 papilla. 



