DEVELOPMENT WITHOUT ACTUAL YOLK-SAC. 267 



giving origin to an external yolk-sac which, however, is not nearly 



so large as in cases already mentioned 



(Fig. 124). As development advances, this _, 



yolk-sac decreases in si/e, and, in mature 



'mbryos, at hatching, there is not any 



trace of it. 



The differences in si/e and shape of body 

 existing between the two sexes of Ar^onanfn, 

 also find their first expression during post- 

 embryonic life. Xo sexual dimorphism 

 could be observed in any of the many 

 embryos examined by us.* This applies 



also to the striking hectocotylised arm of FlG - 124. - Embryo -of 



Argonauta argo, with 

 the male, which in other Cephalopods funnel still incompletely 



also, becomes differentiated only as sexual arms^?!, ey"?"!/*, yolk- 



'' 



milt uritv is o-raduallv attained. Mature sac ' /", michal muscle ; 



///, mantle ; tr, funnel. 



embryos of Octopus, Loligo and Sepia show 



no sign of this modification; this is the less strange as the arms 

 'are still far from being fully developed in these embryos. 



B. Development without actual yolk-sac. 



The description of the above forms may best be followed by that 

 of the Cephalopod observed by GRENACHER (No. 14), a form the 

 systematic position of which has not yet been determined. It 

 probably, however, belongs to the large division of the Oigopsida, 

 perhaps to the genus Tv.nthi*. STREENSTRUP (No. 42) believes 

 that it resembles Ommastrephes (see also p. 236). The eggs, unlike 

 those of the forms hitherto mentioned, are spherical and distinguished 

 by the violet colour of the yolk which elsewhere is yellow. They 

 are about 1 mm. in diameter, and thus much smaller than those 

 of Argunnuta. This is a remarkable fact since, judging from the 

 quantity of spawn and the number of eggs contained in it (p. 236), 

 this Cephalopod is most probably a large animal. The small size 

 of the eggs, and the small quantity of yolk contained in them afford 

 an indication of the manner of development of this form, which is 

 marked by an almost entire absence of the external yolk-sac. 



* The number of eggs laid by the female Argonaut is very large, so that a 

 considerable number of embryos are to be found in the egg-bunches within 

 the shell. The eggs to which we had access belonged -to various females and 

 were at different stages of development, and, although no fully mature embryos 

 were found by us, we were justified in forming the above conclusion from those 

 in a late stage of development. 



