DEVELOPMENT OF THE EMBRYONIC RUDIMENT LOLIGO. 263 



the embryo in rising uj> from the yolk produces a change in the 

 position of the arms which shift from the funnel-side more towards 

 the oral side (Fig. 117-120). 



In Ijli(/<> J'l-fiHi, the three pairs of arms depicted in Fig. 1 15, fir, develop 

 first and only when these have become differentiated, does the pair which 

 lies nearest the funnel appear (BROOKS). The fifth pair seems t-> form very 

 late. The same is said to be the case in 

 ( '.KI.N ACKER'S Cephalopoda!) embryo (p. 271). 



The second pair of arms, the prehensile 

 arms, grow with special rapidity, out- 

 stripping in development not only the first 

 but the other arms also (Figs. 119-121) ; 

 external investigation, however, reveals no 

 special peculiarity in the rudiments of these 

 arms. This corresponds with the adult 

 condition in other more primitive Decapods 

 (Omma*trephfx). Only later, when the 

 embryo hatches, are the prehensile arms 

 distinguished by the fact that the basal 

 part is free from suckers, this part lying in 

 a depression. In younger embryonic stages 

 and even in older stages (Fig. 120 A and 

 /?), five pairs of arms can easily be made 

 out, but the fifth develops much less than 

 the rest, so that in later stages (e.g., in 

 that depicted in Fig. 121) it appears 

 merely as two small cones which can hardly 

 be made out. This fact no doubt led to 

 the assumption that only four pairs of arms 

 appeared as rudiments in the embryo, the 

 fifth forming later. 



Since the yolk-sac occupies a ventral 

 position (between the mouth and the anus), 

 the same position must be ascribed to the 

 arms that surround it. This is especially 

 noticeable in the younger stages (Fig. 



116 B-I)}, although it can also be recognised in older embryos 

 (Fig. 120 A and B). The mouth here still lies outside of the circle 

 of arms, but soon shifts within it (Fig. 120 B} or rather is surrounded 

 by the arms as they shift dorsally. This occurs simultaneously with 

 t hr degeneration of the yolk-sac that now begins. The mouth finally 



FIG. 121. Older eniuryo of 

 Istliiju riil'/i'.i'ix, seen from 

 the posterior or funnel-side 

 (original), a, the arras of 

 the second pair and be- 

 tween them the first pair 

 of arms ; ecu, eyes ; dti, 

 yolk-sac ; .//, fins ; k, gills ; 

 between the two gills is 

 the anal papilla; fr, funnel. 

 The embryo is already- 

 covered with chrornato - 

 phores. 



