S24 F. M. BALFOUR. 



shell which has the shortest pair of strings for attachment 

 This is probably due to the shape of the cavity of the shell, 

 and is certainly not due to the presence of any structures 

 similar to chalazse. 



The Yolk. 



The yolk is not enclosed in any membrane comparable to 

 the vitelline membrane of Birds, but lies freely in a viscid 

 albumen which fills up the egg-capsule. It possesses con- 

 siderable consistency, so that it can be removed into a basin, 

 in spite of the absence of a vitelline membrane, without falling 

 to pieces. This consistency is not merely a property of the 

 yolk-sphere as a whole, but is shared by every individual 

 part of it. 



With the exception of some finely granular matter around 

 the blastoderm, the yolk consists of rather small, elliptical, 

 highly refracting bodies, whose shape is very characteristic 

 and renders them easily recognizable. A number of striae 

 like those of muscle are generally visible on most of the 

 spherules, which give them the appearance of being in the act 

 of breaking up into a series of discs; but whether these striae 

 are normal, or produced by the action of water I have not 

 determined. 



Position of the Blastoderm. 



The blastoderm is always situated, immediately after im- 

 pregnation, near the pole of the yolk which lies close to the 

 end of the egg-capsule. Its position varies a little in the 

 difierent species and is not quite constant in different eggs 

 of the same species. But this general situation is quite 

 invariable. 



Segmentation. 



In a fresh specimen, in which segmentation has only just 

 commenced, the blastoderm or germinal disc appears as a 

 circular disc, distinctly marked off by a dark line from the 

 rest of the yolk. This line, as is proved by sections, is the 

 indication of a very shallow groove. The appearance of 

 sharpness of distinction between the germ and the yolk is 

 further intensified by their marked difference of colour, 

 the germ itself being usually of a darker shade than the 

 remainder of the yolk ; Avhile around its edge, and apparently 

 sharply separated from it by the groove before mentioned, is 

 a ring of a different shade which graduates at its outer border 

 into the normal shade of the yolk. 



These appearances are proved by transverse sections to be 



