M. tJssow'a Zoologico-Emhryological Investigations. 21'' 



The pit-like depression of the upper lamella, which forms 

 the buccal orifice, j^radually 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, i-epresents 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 csecal 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 oesophagus 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 pei'iod, 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'U48 millim, in diameter) originate. The canals which 

 lie upon the auditory vesicles become bent (in tne 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 Metschnikotf in Sepiola (I. c. pp. 49-5^), but differ materially from 

 those of KoUiker (/. c. p. 108). ' .» 



Ann. Jb Mag. N. Hist. Ser. 4. Vol. xv. 15 



