DEVELOPMENT WITHOUT ACTUAL XOLK-SAC. 



267 



giving origin to an external yolk-sac which, however, is nor nearly 



m> large as in cases already mentioned 



(Fig. 124). As development advances, this 



yolk-sac decreases in size, and, in mature 



embryos, at hatching, there is not any 



i race of it. 



The differences in size and shape of body 

 existing between the two sexes of Argonauta, 

 also rind their first expression during posl 

 embryonic life. Xo sexual dimorphism 

 could be observed in any of the many 

 embryos examined by us. This applies 

 also to the strikiug hectocotylised arm of 

 the male, which in other Cephalopods 

 also, becomes differentiated only us sexual 

 maturity is gradually attained. Mature 

 embryos of Octopus, Loligo and Sepia show 

 no siejn of this modification ; This is the less strange as the arms 

 are still far from being fully developed in these embryos. 



Fig. 124. — Embryo of 

 Argonauta argo, with 

 funnel still incompletely 

 developed (original), or, 

 arms ; an, eyes : ds, yolk- 

 sac ; hm, nuchal muscle ; 

 m, mantle ; tr, funnel. 



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 Tuuthis. 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 Argorviuta. 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 l»v an almost entire absence of the external yolk-sac. 



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

 considerable number of embryos are to bo found in bhe 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 Erom those 

 in a late stage of development. 



