LI"FE HISTOEY OF THE LIVEE-FLUKE. 109 



slowly, and with an average warmth of 16° C. occupies two or 

 three months. During the winter no progress is made unless 

 artificial heat is supplied. 



All the eggs under the same conditions, however, do not 

 produce embryos in the same time, a certain number are hatched 

 out on every successive day for some weeks or even months, 

 and at the end of this time some of the eggs may remain in 

 the same condition as when just taken from the liver. No 

 explanation can be discovered in the eggs themselves of the 

 very variable time required for the development of the embryo, 

 but the fact is of much practical importance, for eggs scattered 

 over any damp ground may render it dangerous for a long period. 



The granular character of the secondary yolk-cells render it 

 very difficult to follow in detail the growth of the embryo 

 whilst still within the egg, and as the matter is one of theo- 

 retical importance only, the examination of the development 

 by more elaborate methods has been postponed in favour of 

 matter of more practical interest. I hope, however, to have 

 the opportunity before long of observing the formation of 

 the layers in the embryo. All that can be seen in the egg by 

 direct examination is as follows. The embryo increases in 

 size, being nourished by the absorption of the secondary yolk. 

 The outlines of the yolk-spheres become more distinct and 

 the granules less numerous, whilst some of them appear to 

 coalesce or disintegrate (fig. 2, PI. II). Within them the 

 outline of the embryo is visible, often showing one or more 

 annular constrictions. As the yolk-cells are gradually used up 

 the body of the embryo becomes larger and more plainly 

 visible, and comes to occupy the whole length of the egg (v. 

 fig. 3). Its surface is somewhat bossy owing to the projection 

 of the cells forming the outermost layer of the body. A papilla 

 appears at the anterior end, which is always directed towards 

 the opercular pole of the shell, and a little way behind a 

 quantity of dark brown pigment is produced, giving rise to a 

 double eye-spot, while the surface loses its bossy appearance. 

 Wave-like peristaltic contractions pass along the body from the 

 anterior towards the opposite end. 



