18 ILLINOIS BIOLOGICAL MONOGRAPHS [18 



of the offspring at an early date. The cytoplasm of the germ cells which 

 develop into rediae is granular altho quite transparent. It stains a delicate 

 lavender with Delafield's hematoxylin. On the other hand, from the very 

 outset the cytoplasm of the cercaria type of cell is fibrillar, with many large 

 intermediate vacuoles. It stains a deep magenta with the same dye in the 

 same section as the rediae ova. Figure 46 L represents the first cleavage of the 

 cercaria embryo. The chromosome count is identical to that in each blastomere 

 in a redia-forming embryo. Subsequent divisions are difficult to follow on 

 account of the opacity of the cercaria germ-balls. It is very evident, never- 

 theless, that differentiation of layers and organs takes place much more rapidly 

 in the cercaria ovum than in the redia ovum. 



The argimients produced by Rossbach (1906:433), to show that there are 

 no polar bodies given off by the germ cell, do not hold in the case of Cercaria 

 flabelliformis. The polar bodies have been found not only in cytoplasmic 

 continuity with the ovum, but in the actual state of mitosis preceding the 

 separation of the polar nucleus from the germ ball. Polar bodies are indeed 

 more numerous in the young rediae, since this is the period when the majority 

 of the germ cells free in the body cavity throw off the polar body and mature. 

 Altho Tennent has found three bodies similar to those designated by authors 

 as "polar bodies," no authentic proof is recorded of more than one polar 

 extrusion in the maturing germ cell of a redia or sporocyst. 



In summary, it may be said that the study of the germ cells in the rediae 

 of Cercaria flabelliformis supports the thesis that true parthenogenesis takes 

 place here; that the germ cells are traceable to a mesodermal cell mass in the 

 region of the blind end of the gut; that a single polar body is extruded; and that 

 maturation takes place without reduction. 



It is not surprising that the details of the germ layers have not been worked 

 out in the fertilized trematode egg, because of the yolk inclusions which ob- 

 scure developmental stages and no doubt modify the behavior of the segment- 

 ing cells. Yet it is regrettable that no attempt at the precise origin of the 

 germinal layers has been made on germ balls within the sporocyst or redia. 

 Without any effort at this exact study of the problem the writer has followed 

 in the living rediae of Cercaria pellucida and C. konadensis, and in the sporo- 

 cysts of C. dendritica the development of the germ balls from the single mature 

 ova, thru unequal divisions into two, three and five cells, up to the morula 

 stage. 



PARTHENITAE (SPOROCYST AND REDIA) 



Since the classic work of Thomas (1883) on the life-history of Fasciola 

 hepatica, it has been the common custom to define the sporocyst and redia 

 in terms of stages in the life-history of the trematode. The sporocyst is the 

 metamorphosed miracidium, and the redia arises within the sporocyst. The 

 cercaria is the parthenogenetic offspring of the redia and develops into the 

 adult trematode. While this represents a so-called typical life-history, it is 



