M. Ussow's Zoologico-Emhryological Investigations. lOl 



(hletii^ Leacli, and Loligo sagittata^ Lam.) and one Octo})od 

 (Argonauta ArgOy Linn.). 



For tlic more convenient exposition of the facts discovered 

 by me, I .shall divide this communication into two halves. 1. 

 Anatomico-physiological data relating to the structure of the 

 female generative organs and the mode of formation of the ova*. 

 2. The results of my embryological investigations upon : — 

 «, the process of segmentation ; b, the formation of the blasto- 

 derm and the production of the germ-lamella3 (first period ot 

 development) ; and c, the original foundation of the organs up 

 to the appearance of the typical Cephalopodal formf (second 

 period of development). As the development of the above- 

 mentioned Cephalopoda is verg concordant in essential points, 

 I shall not describe the development of the individual species, 

 but the course of development in all the four species, in order 

 to be as concise as possible. 



L The structure of the ovaries and the mode of formation of 

 ike ova of the Cefphalopoda. 



In youngish female individuals of various species of Cepha- 

 lopoda, the unpaired, rather large ovary, enclosed in the peri- 

 toneal sac, and situated in the lower, narrower part of the 

 mantle, consists of numerous caecal, ramifying tubules, which 

 form its glandular parenchyma. In general the structure of 

 the ovary is like that of the ovary in the Vertebrata, especially 

 in Birds and Tortoises |. There may be distinguished in it : — 

 a, the very thin sheath {theca folUt^.li)^ consisting of fibrous 

 connective tissue ; ^, the internal, one-layered epithelial mem- 

 brane {memhrana granulosa), which lines the inner surfaces 

 of the above-mentioned tubular and vesicular ovarian spaces 

 quite uniformly. In the fii'st of the above-mentioned coats 

 ramifies the thin artery (genital artery), which takes its origin 

 from the lower part of the ventricle. The Graafian follicles 

 are formed at different periods of the spawning (as may be 



* Besides the above-mentioned species, I have investigated the mode 

 of formation of the ova and some stagey: of development in Ommastrephea 

 todarus, Russia 7naerosoma, and Sepia biseriitlis, Mxtntf. 



t I am at present occupied with the study of the last period of deve- 

 lopment of the Cephalopoda, namely the development of the embrto, 

 which I am enabled to do by means of a great store of remarkably well- 

 preserved material. The production of the organs is of especial im- 

 portance in comparative embr\'ology ; and to this I have particularly 

 directed my attention. 



X As made known by Gegenbaur's investigations (Ai-ch. fur Anat. &c. 

 1861, p. 4!Jl); Hiss, Erste Anlage der Wirbelth. p. 19 et seq. pi. ii. ; and 

 Waldeier, Der Eierstock, pp. 48 & 69, pi. iv. 



