M. Ussow's Zoologico-Einhryolo(jical Investiyations. 105 



regard to the spawning-time*, the nuniLcr of mature ova and 

 other details in the formation and development of the Graatian 

 follicle and other accessory glandular organs (albumen- or 

 nidimental glands of the Cephalopoda), 1 reserve their de- 

 scription for a complete memoir on the animals named. 



II. Segmentation of the ova of the Cephalopoda^ and forma- 

 tion of the one-layered germ {blastoderm). 



The whole of the mature ova which fall from the Graafian 

 follicles into the ventral cavity are, apparently, without ex- 

 ception, fccundatedf. The mature Cephalopod ovum, which, 

 in form, is very like a hen's egg, contains the following 

 parts : — 1, a very small mass of the so-called formative vitellus, 

 which, as we have seen, represents the finely gTanular proto- 

 plasm of the primitive ovicell with its nucleus (germinal 

 vesicle) ; 2, a greater or less quantity of the rather viscous, 

 fatty nutritive vitellus ; 3, a perfectly transparent albuminous 

 substance which occupies the space between the vitellus and, 

 4, the many-layered vitelline membrane (chorion) with its 

 tubuliform micropyle ; and, lastly, 5, a more or less thick, 

 many-layered egg-capsule which sometimes runs out into an 

 elastic thread, serving to attach the ova to various objects 

 under water {Argonauta\^ Sepia) , and sometimes forms a 

 longer or shorter sac containing 10-100 or more ova {Sepiola^ 

 Loligo). 



At the moment of fecimdation the germinal vescicle does 

 not disappear ; and the segmentation of the finely granular 

 protoplasm of the primitive ovicell, or the so-called formative 

 vitellus, which may easily be distinguished by its somewhat 

 dark coloration from the nutritive vitellus, always com- 

 mences with the cleavage of the germinal vesicle. In Argo- 

 nauta the process of segmentation takes place chiefly in the 

 body of the mother, and, indeed, during the movement of the 

 ova in the toituous oviducts § ; whilst in all the other Cepha- 

 lopoda observed by me the segmentation always seems to 



* In Argonauta the spawning-time lasts from May to August, in Loligo, 

 SepioUt, and Ommastrephes from March to June ; but I obtained mature 

 ova of Sejna in Naples almost all the year round, except in August. 



t Among the thousands of Cephalopod ova which I have examined, 

 scarcely any unfecundated ova occurred. 



X In Aryonuuta to the apex of the shell, so that the female, which is 

 seated in the shell, covers with her hinder parts the racemose gi'oups of 

 eggs placed within the spire. 



§ 1 observed the first stage of segmentation in ova which were taken 

 out of the entrance to the oviducts, whilst in those near the witice, eight, 

 or even sixteen, segments are already present. 



