602 L- R- Caky, 



could be recognized as anterior or posterior. In sections (Fig-. 9) 

 the body wall is seen to be made up of a very thin cuticular layer 

 immediately beneath which there are a number of muscle fibres, 

 composing two systems. When a well defined axis becomes recogniz- 

 able these two sets of muscles are seen to be longitudinal and 

 circular respectively. Inside the muscular layers there is a layer 

 of cytoplasm containing a number of nuclei, but there are as yet 

 no cell walls. The body cavity contains germ cells (eggs) in early 

 stages of development, 



In somewhat later stages the sporocyst acquires the characte- 

 ristic form shown in (Fig. 11). The broad end — posterior — always 

 contains the ovary (Keimlager) when this has become differentiated. 

 In some instances the older sporocysts are surrounded by a more or 

 less well defined layer of cells, the "Paletot" of early writers. My 

 material clearly shows that, as has been pointed out by Schwarze 

 (1886), this layer is made up of the leucocytes of the snail and is 

 not a part of the sporocyst. In some instances there is some evidence 

 that a division of the sporocyst is taking place; but not a single 

 instance has been observed where this could be determined with 

 certainty. 



Sections through the wall of older sporocysts show the outer 

 layers in practically the same relation as that described for the 

 earlier stages. In the structure of the inner layer, however, some 

 very important changes have taken place. These will be described 

 in detail in a following section of this paper treating of the origin 

 of the parthenogenetic eggs. 



2. The redia. 



The youngest rediae found were about 0,5 mm long and 0,2 mm 

 in diameter. These had already attained the characteristic shape 

 of the redia, and the pharynx, gut, and body cavity were already 

 marked out, though not definitely established. The mature rediae 

 (Fig. 4) are about 1 mm long and 0,3 mm in diameter. The anterior 

 end is broadly rounded, while the posterior end is sharply pointed. 

 This part of the body is very active during life, often becoming 

 drawn out into a narrow process which is continually moving from 

 side to side. At a point about one fifth the length of the body from 

 the posterior end there is on each side of the body a lateral out- 

 growth; the so called „posterior locomotor appendages". 



These last mentioned structures, which are always present in 



