104 M. Ussow's Zoologico-Emhryohgical Investigations. 



it acquires the form of a very flat, conical disk, in the thick- 

 ened central part of which the germinal vesicle is situated. 

 The above-described folds of the grauuJosa are gradually 

 effaced as the ovum enlarges, and finally disappear alto- 

 gether, so that at last it becomes perfectly smooth both within 

 and without. The perfectly mature ovum by its own weight 

 ruptures the very thin part of the envelope (the so-called 

 stigma) at the upper pole, and is fecundated* at the moment 

 when it falls into the ventral cavity [Argonanta) . 



For the elucidation of the above-mentioned question as to 

 the continual development of the ova of Cephalopoda com- 

 mencing at difterent times, I may add that 1 have succeeded 

 in observing that, at the time of the strongest development of 

 the folds, new ova are developed in the Graafian follicle from 

 any of the cells of the epithelial membrane. A part of the 

 inner surface of the fold gradually covers the newly formed 

 ovum, which during its enlargement protrudes at the surface, 

 becomes constricted off from the Graaiian follicle, and finally 

 remains united to the theca foUiculi only by means of a short 

 peduncle. Thus, in consequence of more or less coj^ious 

 nutrition, the cells of the granulosa of a Graafian follicle may 

 in a short lapse of time bring the primitive ovicell to full de- 

 velopment, and secrete the whole mass of the nutritive vitellus 

 and finally the transparent chorion f. This is in its main 

 features the mode of formation of the Graafian follicle and 

 the ova of the Cephalopoda. Of its coiTCCtness I am perfectly 

 convinced by an attentive and frequently repeated study of 

 the process. As regards the original development of the 

 female sexual organs of the Cephalopoda, I could not trace it, 

 as it appears to be correct that the mature embryo, after 

 its escape from the egg, and even the yoimg animal from one 

 to three days old, possess no trace of these organs J ; but at the 

 end of three days, during which it has used up the -ivhole of 

 the outer and a part of the inner nutritive vitellus, the animal 

 dies, and consequently deprives us of all possibility' of inves- 

 tigating the development of the sexual organs and the })ail 

 taken by the germ-lamellae in their construction. "W'^ith 



* I can assert this positively with regard to the fecundation of Argo- 

 nauta. Although in all the other species, also, 1 found perfectly mature 

 epcnnatophora in the ovaries, the segmentation takes place always outside 

 the body, which woidd indicate a pause between the fecundation and the 

 commencement of development. 



t In Aryonaiita also the longer or shorter filifoi-m process. 



X The same results were obtained also by Kolliker (I.e. p. 110) and 

 Metschnikoff(/. c. p. (Jo). At the end of the third period I have observed 

 below the ventricle in Sepia and Lnlif/o an aggregation of cells, from 

 which it is possible the sexual organs are developed. 



