2S6 ERNEST WAUKEN. 



by several rows of cells (fig. 10, e. c), and often the cyst 

 encloses all the cells, so that none are excluded from further 

 development. 



Cercaria Stage. — The embryo enclosed by the thick cyst 

 will produce the young Distomum, and it may perhaps be 

 regarded as a cercaria, and the cyst as a cercaria-cyst. 

 Scattered through the body there can be seen small clumps 

 of deeply staining cells. After the cercaria-cyst is completely 

 formed they seem to disappear, and I believe they are cystogen 

 p-lands which secrete the substance which forms the thick cyst 

 (fig. 10, cy. c). Further differentiation now sets in. Around 

 the transversely placed tubular structure above described the 

 cells of the embryo become elongated and stringy in appear- 

 ance (fig. 10, m. c. s.), and they do not stain at all readily 

 Avith htematoxylin. The fundament thus produced will form 

 the muscular cirrus-sac and the structures which lie within. 



Reference to fig. 10 will show that there is still no differ- 

 entiation of an outer layer into an ectoderm, neither has the 

 fundainent of the gut appeared. 



Body Covering and Cuticle. — The outermost cells of 

 the cercaria never form an epithelium, for they are quite 

 irregularly disposed, and are indistinguishable from the cells 

 below. These outermost cells, however, now fuse together, 

 and produce a thick outer layer of substance that stains like 

 protoplasm. The nuclei of the cells are not included in this 

 outer layer of protoplasm, but lie either just on the inner 

 side of it or are connected with it by strands of protoplasm 

 (Fig. XIV). This outer layer of modified protoplasm may be 

 called cortex, and it is from this that the future cuticle and 

 spinelets are produced (Fig. XV). 



The remainder of the cortical substance (Fig. XV, r. cor.) , 

 after the cuticle is formed, would appear to persist as the 

 thin layer of protoplasm which may be observed in the adult 

 immediately below the cuticle (Fig. XYI,p. I.), and connected 

 with the cortical cells by means of branching strands of pro- 

 toplasm. These cortical cells are the so-called epidermal 

 cells {ep. c). 



