346 KOREN AND DANIELSSEN ON THE 



single ovum, we may return to the embryos formed by multiple 

 ova, and describe at length their further development. 



We have already remarked, that after the formation of the 

 ciliary membrane, the foot and the two rounded lobes are the 

 organs first developed. At about the same time we perceive a 

 transparent mass between the membranes and the conglomerate 

 ova (fig. 28 d). Cells are developed in this mass in layers, and 

 give rise to the mantle (figs. 29, 30Z>). The most sloping part of 

 the latter secretes a very clear and viscid humour, which increases 

 by degrees and forms the rudiment of the shell, which, when it 

 first appears, resembles a clear and gelatinous pellicle, wherein 

 subsequently calcareous particles are deposited (fig. 29 a). These 

 particles afterwards increase in number and impede the exami- 

 nation of embryos which are a little older. 



The lobes are small at first, but their volume rapidly increases; 

 a multitude of cilia appear on the surface, cirrhi are developed 

 from their upper edge and produce much more vivacious move- 

 ments (figs. 29 d } 30 e). The foot becomes strongly separated 

 on the ventral surface and thus forms a transverse eminence 

 (fig. 28 b), which rapidly increases in volume and exhibits the 

 first rudiments of the auditory organs at its base, developed as 

 in Buccinum undatum (figs. 29 e, 30/). At the same time as the 

 auditory organs, the rudiments of the tentacles, of the eyes and 

 of the salivary gland, appear. The tentacles commence as two 

 conical eminences, at the base of which the eyes are visible as 

 two rounded vesicles, filled internally with a pellucid liquid \ 

 dark pigment granules may be seen in them (fig. 31 /, m). We 

 have been unable, at this stage of development, to discover any 

 lens, nor have we met with any cilia on the internal parietes of 

 the vesicle. 



The first trace of the salivary glands which manifests itself, 

 is a mass of rounded cells upon each side of the base of the foot, 

 which are usually nucleated. These cells soon acquire a delicate 

 membrane, which afterwards elongates to meet the future oeso- 

 phagus, whose outlines are not yet distinguishable. In pro- 

 portion as the salivary glands are developed, these cells become 

 more and more multiplied in their interior and are closely dis- 

 posed in elongated lines ; we see also, in the widest portion of this 

 organ, a mass of dark yellow pigment granules. In its more 



