100 M. Ussow's Zoologico-Emhryological Investigations. 



distinguished observer unfortunately could neither subject the 

 results obtained by Kijlliker to a thorough testing, nor de- 

 scribe exactly the production of the second gerni-laniella 

 (parcnchjTuatous lamella*), nor, finally, ascertain the mode of 

 fornuition of the intestino-glandular lamella. Undoubtedly 

 i\Ietschnikoti"s observations on the mode of formation of the 

 organs of sight and hearing, so su])erHcially and inaccurately 

 described by his predecessors, are of great value. 



During my long residence in Naples and ]\Iessina I set 

 myself, as one of my principal tasks, to investigate as com- 

 pletely as possible the development of several species of Cepha- 

 lopoda, or, in other words, to subject all previous observations 

 relating to this subject to a careful examination, in order, as 

 far as possible, to enlarge our exceedingly defective knowledge 

 of the embryology of these interesting animals. By the 

 direct observation of living embryos in various stages, by the 

 employment of the most serviceable method of the comparative 

 examination of different sections, and, lastly, by the investiga- 

 tion of a formative vitellus (which would afterwards be con- 

 verted into the so-called germinal spot and then into the 

 embryo) with its parts firmly united, sejiarated in a particular 

 manner t from the nutritive vitellus, I have been able to 

 follow step by step the whole develo})mental cycle of several 

 forms of Cephalopoda. Some difficulties, which met me in 

 this little-followed method of investigation, are fully compen- 

 sated by a series of new and interesting facts, repeatedly con- 

 firmed by me, which I have succeeded in discovering and 

 elucidating. 



I have already succeeded in observing pretty accurately the 

 development of the embryo in four species of Cephalopoda, 

 namely three Decapods {Sejna officinalis, Linn., Sepiola Ron- 



* Loc. cit. p. G7. 



t In general terms this method is as follows : — First of all the fecun- 

 dated ovum, with its capsule, is laid for from five toten minutes in a weak 

 solution of chromic acid, in which the capsule is removed. Then the 

 ovum is placed for two or three minutes in fi'esh water, mixed with two 

 or three drops of acetic acid. The chorion is removed in another portion 

 of fresh water. The viscid, semifluid nutritive \-itellus inmiediately flows 

 out, while the germ, which is already somewhat hardened, falls to the 

 bottom of the watch-glass. After the removal of the water the germ is 

 carefully spread upon a glass slide, and, after being coloured with carmine, 

 mounted in glycerine. The foundation of this wliole operation is that the 

 germ hardens more quickly than the peripheral layer of the nutritive 

 vitellus ; for only in this case will tlie former separate from all inversions 

 of the latter. Hy its aid I have succeeded in separating the formative 

 vitellus from the imlritive vitellus. tlie whole of wliich it suiTounded, 

 and in making a considerable collection of preparations of various stages 

 of development of the Cephalopoda. 



