364 Messrs. Koren and Danielssen on the 



a small, clear nucleus. After the formation of these, we distin- 

 guish 4 smaller spheres, which are clear and furnished with a 

 similar clear nucleus. These latter spheres have a cellular ap- 

 pearance. J. Miiller found it impossible to explain how these 

 little spheres issue from the large spheres of segmentation. 

 These 4 clear spheres multiply rapidly, however ; their number 

 may reach 8, 16, or even more, whilst the 4 large opake spheres 

 remain inactive. Even after the cilia have made their appear- 

 ance round the cellular peripheric layer, the 4 large spheres of 

 segmentation have undergone no change. 



Gegenbaur has also observed that in the Pteropoda the ger- 

 minal vesicle divides, and that it forms first 2 and then 4 large 

 spheres of segmentation, of which one of the latter again divides 

 into two. He thinks that the peripheric layer is formed by one 

 of the 4 spheres of segmentation which has become changed 

 into a layer of clear cells. The 3 central spheres also remain 

 inactive at first. In Hyalea tridentata one of the 2 spheres 

 of segmentation divides into 2 smaller ones, and these into 2 

 others, until a mass of clear cells is formed surrounding the in- 

 active sphere of segmentation. It sometimes ha])pens also that 

 this divides into 2 equal spheres. 



Another modification observed in regard to the process of 

 segmentation in Hyalea is, that the vitellus divides into 2 un- 

 equal spheres of segmentation, the largest of which again divides 

 into 2, producing 3 equal spheres. One of these also divides 

 into several smaller spheres, and forms a peripheric layer which 

 surrounds the 2 inactive spheres. 



Thus we see that, even in the same species, the segmentation 

 may undergo considerable alterations. We have already seen 

 that in Buccinum nndaium the germinal vesicle disappears, and 

 that there is no clear body in the spheres of segmentation. 

 We have also remarked, that although the greyish, transparent 

 mass already mentioned is exuded in various stages of segmenta- 

 tion, this nevertheless occurs most frequently when the vitellus 

 is divided into 4 or 8 segments. This exuded mass must be 

 regarded as the peripheric layer already mentioned, for we soon 

 observe in it a cellular structure which forms the rudiments of 

 some external organs, — the rotatory organs and the foot, — whilst 

 the central part and the true spheres of segmentation appear 

 to remain long without alteration (PI. XVII. figs. 5-12). 



In describing the ordinary development oi Buccinum undatum, 

 we have endeavoured to explain clearly the Avay in which the 

 organs are developed ; and as the embryos Avhich issue from a 

 single egg do not appear to present any modification of develop- 

 ment, it would only give rise to a repetition if we were to de- 

 scribe the formation of the organs here. 



