1920] j Ji nson ; Life Cycle of Echinostoma Rcvolutum (Froelich) 339 



other, making a medium the nature of which is difficult to show in a 

 drawing. As the number of cells of the miracidium increases the 

 granules of thp yolk material collect into larger units, but again 

 decrease in prominence until by the twelfth day they have almost dis- 

 appeared. By the seventh day the oily material has formed many well 

 defined globules (fig. 7). On the second or third day the eight, ten, 

 and twelve cell stages are reached (fig. 4). On the fourth day about 

 twenty cells are represented (fig. 5). On the sixth day the number 

 of cells have increased to about thirty (fig. 6), each cell being some- 

 what smaller than in the stages where there are fewer cells. During 

 this time the embryo remains quite distinct from the yolk mass, 

 although the exact boundary line is sometimes difficult to determine. 

 On the seventh day (fig. 7) angular epithelial cells of the miracidium 

 are clearly distinguishable. On the eighth day (fig. 8), the condition 

 differs from the preceding day principally in the great reduction of 

 the granules of the yolk material. Also on this day the epithelial cells 

 are less distinct. On the tenth day (fig. 9), the embryo has attained 

 a shape which is somewhat like that of a fully developed miracidium, 

 the nuclei of the epithelial cells are less distinct and the oily globules 

 are fewer but larger in size. On the eleventh day (fig. 10), the 

 embryo is still longer and on the posterior end a tail-like structure 

 appears which possibly represents the region of the primitive germ 

 balls. This projection disappears in a few days. The first movement 

 of the embryo is also to be seen on this day. which consists of contrac- 

 tion and expansion at the anterior end. On the twelfth day (fig. 11), 

 the movement has increased, while the muscular nature of the body 

 wall has become evident. The oily globules are still fewer in number 

 yet larger. At. the end of two weeks (fig. 12), germ balls can be seen 

 at the posterior end of the embryo miracidium, while the activity has 

 still further increased. On the sixteenth day (fig. 13), the embryo is 

 nearly as long as the egg, the eye spot has appeared and more germ 

 balls are visible. For the first time flame cells are to be seen. They 

 are two in number, one being anterior to the other. Also the primitive 

 digestive tract is visible. On the seventeenth day (fig. 14), the germ 

 balls have increased to about twelve or fifteen in number, the primitive 

 digestive tract is more clearly outlined, the epithelial cells are becom- 

 ing quite opaque and thus the flame cells can hardly be seen. The 

 activity of the miracidium has also greatly increased. On the twentieth 

 day (fig. 15), the embryo has greatly increased in length, being now 

 about 0.120 mm., but this is accompanied by a decrease in width. 



