212. M. Ussow’s Zoologico-Embryological Investigations. 
median line * of the embryo, a rather inconsiderable promi- 
nence, which, like most of the outgrowths, proceeds from the 
second germ-lamella (in this case chiefly from the intestino- 
fibrous layer), and is also covered by the upper lamella. This 
prominence forms the first commencement of the anus. About 
the same time, in all the Cephalopoda investigated by me, the 
four (Argonauta) or five pairs of rudimentary arms make their 
appearance very rapidly after one another (at the utmost in 
two days; in some the first three pairs simultaneously {). 
This seems to confirm Van Beneden’sf observation, which was 
rejected by Kélliker § and afterwards by Metschnikoff ||. The 
rudiments of the arms are developed as hemispherical 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) 
concentric series of large but flat cells, constricted off from 
the segments after the meridional segmentation, and at first 
lying scattered in isolated groups 4]. 
On the third day of the second period the rudiments of the 
auditory 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 are 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 surface. The walls of the 
latter soon become thicker in many parts **. 
* In the longitudinal line which passes through the buccal aperture and 
the middle of the mantle, and divides the embryo into two symmetrical 
halves. 
+ In Loligo, Sepiola, and Argonauta. t Loe. cit. p. 7, fig. 9. 
§ Loe. cit. p. 60. || Loe. cit. p. 55. 
q 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 each auditory vesicle 
(0°32 millim. in diameter in Zo/igo), on its upper wall, shining granules 
