SOME POINTS IN DEVELOPMENT OF SCORPIO PULVIPES. 591 



the end furthest from the ovarian tube), and at the posterior 

 end into the space in the middle of the embryo described 

 above. It is certainly extraordinary that the stomodseum 

 should be the first structure developed, but the reason will be 

 found in the peculiar mode of nutrition of the embryo in its 

 later stages. 



The outermost layer of the embryo (fig. 7, am.) is 

 separated from the rest by a distinct space extending all 

 round. It represents the so-called amnion, which is so well 

 developed in Euscorpius, but in this form appears to be 

 confined to the earlier stages, as I have been unable to find 

 any trace of it in the later ones. A protective envelope of 

 this sort is not so necessary in this form, where the embryo 

 remains in situ till it has attained its full growth, as it is in 

 a form like Euscorpius, where the eggs pass into the ovarian 

 tube at an early stage, and its slight development and early 

 disappearance are probably due to this change of habit in the 

 embryo. Its presence seems to point to the condition in 

 Euscorpius being the more primitive, a conclusion which is 

 supported by many other facts in the development. 



In the next stage, in which the embryo has increased 

 very considerably in length, the body-wall (fig. 9, ep) is 

 very thin, and composed of flattened cells on the dorsal 

 and lateral surfaces ; while on the ventral surface, where 

 the mesoblast and nervous system will afterwards make their 

 appearance, it is formed of two or three layers of cells with 

 large round nuclei. 



The gut is not yet formed, but its position is shown by a 

 large cylindrical mass of yolk, the central portion of which 

 has a curiously honeycombed appearance. The rest of the 

 body space is filled with yolk spheres, the material for the 

 formation of which must have been derived from the cells of 

 the diverticulum. 



The cavity of the diverticulum beyond the point to which 

 the embryo extends is full of a finely granular substance 

 secreted by the cells which line it. The granular structure 

 may be due to coagulation of a fluid by alcohol. Here and 



