212 M. Ussow's Zoologico-En^i'yological Investigations. 



median line * of the embiyo, a rather inconsiderable promi- 

 nence, which, like most of the oiitgrowtiis, proceeds from the 

 second germ-lamella (in this case chietly from the intestino- 

 fibrous layer), and is also covered by the u))per lamella. This 

 prominence forms the first commencement of iXiaanus. xibout 

 the same time, in all the Cephalopoda investigated by me, the 

 four [Argonauta] or five j)airs of rudimentary arms make their 

 appearance very ra})idly after one another (at the utmost in 

 two days ; in some the first three pairs simultaneously f). 

 This seems to confirm Van Beneden's j observation, which was 

 rejected by Kolliker § and afterwards by Metschnikoff i!. The 

 rudiments of the arms are developed as hemis|)licrical out- 

 growths, composed chiefly of the dermo-muscular layer and 

 covered by cells of the blastoderm. They all make their 

 appearance on the annular part of the germinal disk situated 

 on the equator, which is formed by several (three or four) 

 concenti'ic series of large but flat cells, constricted off from 

 the segments after the meridional segmentation, and at first 

 lying scattered in isolated groups 1|. 



On the third day of the second period the rudiments of the 

 auditor}/ organs^ the pharynx^ the salivary glands, the anal 

 orifice, and the external fold of the eye-ovals are added to 

 the organs already enumerated and now undergoing further 

 development. 



Between the outer margin of the rudiment of the funnel 

 (at the part where its cartilages, although indistinctly, are 

 beginning to be formed) and the commencement of the anterior 

 cephalic lobe the upper lamella becomes a little depressed on 

 both sides of the ventral surface of the embryo, and forms two 

 (at first very small) pits, which ai-^ sharply marked in both 

 longitudinal and transverse sections, and represent the rudi- 

 ments of the auditory organs, only approaching each other at 

 the close of the third period. Their trumpet-like peduncles, 

 which at the end of the second period are entirely constricted 

 off from the upper lamella, become converted into canals, 

 which finally lie upon the auditory vesicle, which is com- 

 pletely separated from the outer surfiice. The walls of the 

 latter soon become thicker in many parts **. 



• In the longitudinal line which passes t]irou<rh the buccal aperture and 

 the middle of the mantle, and divides the embryo into two symmetrical 

 halves. 



t In Loligo, Sepwla, and Argonauta. X Loc, cit. p. 7, fig. 9. 



§ Loc. cit. p. 60. II Loc. cit. p. 35. 



'% See description of the process of segmentation. 



•• At the beginning of the third period, in all the Cephalopoda investi- 

 gated by me, there are formed in the cavity of eacu auditory vesicle 

 (0-;42 millim. in diameter in Loliffo), on its upper wall, shining granules 



