G7 



features of the development are however the same, as will be 

 gathered from the following description: — The ripe egg con- 

 sists inside the egg canal of the reddish-brown yelk mass, 

 the soft egg membrane, and lastly these accessary structures, 

 namely, on the inner surface, a simple layer of small yellow 

 cells, and outside a regular layer of long, blunt oval-shaped 

 tufts joined one to another at the base. Both structures 

 exist already in the ovarium. The yellow cells during deve- 

 lo]3ment approach nearer the embryo, and form, with a 

 gelatinous layer Avhich arises between it and the embryo, the 

 mantel. The mantel is, therefore, a persistent egg envelope, 

 which is formed in the ovarium and derives no elements 

 from the yelk. The fertilisation of the yelk takes place when 

 free, after the egg has been laid. A cleavage cavity (Fur- 

 chunghohle) is observable as soon as thirty-two cleavage 

 masses have been formed ; towards the end of the cleavage 

 process I cannot detect it any longer ; it appears at last to 

 be filled up by the increased number of cleavage masses, so 

 that its persistence as the body cavity, that is, as a narrow 

 interspace between the outer skin and the alimentary layer, 

 cannot be considered as proved. The formation of the ali- 

 mentary canal I saw exactly as Kowalevsky describes it ; the 

 spherical cleft-yelk-mass bends in like a cup, the cavity of 

 the cuj) becomes the alimentary tract ; the uppermost simple 

 layer of cells are marked off on its walls from the deeper 

 ones by a division, and become the foundation of the outer 

 skin ; the inner layer becomes the alimentary wall. There 

 lies between this and the outer skin, when the cup is crescent- 

 shaped, yet a third layer, as I can distinctly distinguish in 

 my specimens. 



The hemispherical cup tends now again to take the shape 

 of a ball, narrowing towards the mouth, which later on closes 

 itself entirely. Before this closure is effected the nervous 

 system has become shaped out as a fusiform cavity, and the 

 foundation of the tail separates itself from the main body, 

 which contains the alimentary cavity and nervous cavity. I 

 must here explain the singularities of the beginning of the 

 formation of the nervous system ; it is not quite clear to me 

 Avhether the outer sl^'in plays a part in it or not. This part 

 of the examination presents extreme difficulties, oecause the 

 transparent space formed by the cavity is still very narrow, 

 whilst the rudiments of other organs are packed closely toge- 

 ther, and the connection of the cells is still too loose to bear 

 much pressure from the covering glass. Everything appears 

 much clearer at a period soon after this, when the embryo 

 has assumed a pear shape. It lies then in a bent-iip posi- 



