M. Ussow’s Zoologico-Embryological Investigations. 212 
The pit-like depression of the upper lamella, which forms 
the buccal orifice, gradually penetrates deeper (between the 
two layers of fusiform cells of the intestino-fibrous layer, 
which lies between the nutritive vitellus and the dermo- 
muscular layer, which bound it) ; and at the bottom of this 
shallow pouch-like pit.there is formed a small prominence 
composed of cells of the middle lamella (dermo-muscular 
layer). This prominence, which lies to one side at the 
hinder wall of the pit, and, like this, is covered by cells of 
the upper lamella, represents the hinder part of the pharynx, 
and becomes subsequently (in the third period) converted into 
the so-called organ of taste, with its muscular tissue and un- 
cinate radula. Between the hinder wall of the original buccal 
cavity and the above-mentioned prominence the upper cell- 
layer of the latter closes into a thin and short cecal tube. 
This tube lengthens pretty rapidly and then becomes forked, 
and thus forms the rudiment of the efferent duct of the salivary 
glands, which are developed (in the third period) at the ends 
of the two branches of the above-mentioned tube. The original 
funnel-shaped tube (wider above), however, represents in its 
upper part the rudiment of the buccal cavity, and in its lower 
part that of the wsophagus or anterior intestine. The other 
parts of the pharynx, the lower and upper jaws, and the thick 
muscle of the latter are developed in the third period—the 
jaws as a chitinous secretion of the epithelial envelope of the 
buccal cavity, and the muscle as a thickening of the dermo- 
muscular layer which is applied to the anterior wall of the 
original buccal pit. 
The change which takes place on this (third) day in the 
anal prominence consists in the cells of the upper lamella 
forming in its centre an increasing depression, which is the 
rudiment of the anal aperture. 
Over each of the thickened eye-ovals forming the primitive 
retina appears a fold, consisting of cells of the upper lamella, 
which grows rapidly, and covers the whole of the oval at the 
end of this second period, although a small aperture remains 
in the centre of the fold. At the same time small yellow 
(0:04 millim. in diameter), which soon unite together, consisting of a 
calcareous secretion from the cylindrical epithelial cells ; and from these 
the two otoliths (0-048 millim, in diameter) originate. The canals which 
lie upon the auditory vesicles become bent (in the third period), and their 
internal epithelial walls covered with cilia. In general my observations 
on the development of the auditory organs agree with the results obtained 
by Metschnikoff in Sepiola (J. c. pp. 49-53), but differ materially from 
those of Kolliker (/. ¢, p. 168). 
Ann. & Mag. N. Hist. Ser. 4. Vol. xv. 15 
