126 A. p. THOMAS. 



Further growth in size is accompanied by a change in shape, 

 and it then becomes possible to distinguish between the germs 

 destined to become redise or cercarise. The growth of the 

 young redia within the redia agrees in every respect with the 

 development of the mother-redia within the sporocyst. The 

 growth of the cercaria follows a different line. 



It may be asked what determines the character of the 

 progeny, whether the germ shall become redia or cercaria. 

 My observations are not suflBciently extensive to definitively 

 decide the question, but it appears to me that the season of 

 the year is one of the principal determining causes. Redise 

 producing redise were only found during warm weather, in the 

 cold months cercarise were always produced directly. Further, 

 it is a noteworthy fact that I found at the beginning of the 

 autumn a redia, containing a single daughter-redia in addition 

 to numerous cercarise and their germs (fig. 13). I am inclined 

 to think that the redia was producing rediae but that a fall of 

 temperature induced the formation of cercarise instead. The 

 explanation suggested is the more likely to be correct, since 

 such an arrangement would be highly advantageous to the 

 species. 



C. 1. The development of the cercaria within the redia. 

 — The earliest stages of the development have already been 

 described up to the time when the germ is an oval mass of 

 cells. As this continues to increase in size it assumes a more 

 elongated shape, whilst one end becomes rather more attenuated 

 than the other. The more slender end becomes slightly con- 

 stricted off to form the rudiment of the tail, which as yet is 

 very stumpy. The remainder of the germ forms the body of 

 the cercaria, it becomes more depressed in shape, whilst cells 

 are separated at the anterior end to form an oral sucker, in the 

 midst of which opens the mouth, and in the centre of the 

 inferior surface to form a ventral sucker equal in size to the oral. 

 The digestive tract is now visible as a solid mass of cells. 

 Immediately following the oral sucker is the rounded pharyn- 

 geal bulb. Then comes a narrow oesophagus ascending sliglitly 

 towards the dorsal surface, and at a short distance in front of 



