16 ILLINOIS BIOLOGICAL MONOGRAPHS [16 



anterior part of the mother rediae large germ balls of both rediae and cercariae 

 are present. The germinal epithelium is confined to the posterior fourth of 

 the wall lining the body cavity. In some of the larval rediae within the 

 mother rediae the earlier stages of the history of the germ cells have been 

 studied. This has enabled the writer to secure a series of stages of the germ 

 cells all the way from the probable derivation of the mesoderm tissue from 

 the base of the gut up thru maturation and segmentation. 



At a stage in the development of the redia when the archenteron is repre- 

 sented by about eight or ten large vesicular cells (Fig. 45), certain cells are 

 found wandering out from the bUnd end of the gut and spreading thru the 

 body cavity. Some of these cells come to he against the wall of the cavity 

 and at first appear as protrusions of the wall; later they seem to constitute a 

 loosely formed inner layer of the wall. Other cells of this type are found free 

 in the body cavity. The majority of these cells that have wandered out from 

 the base of the gut are oval in shape, and have attached to them on one side 

 a small, nodular protrusion, consisting of the film of cytoplasm around a densely 

 granular nucleus. 



All of these cells, whether attached to the body wall or not, are to be re- 

 garded as germ cells, based on their present structure and future behavior. 

 The small nodules are polar bodies. Figure 46 H shows this body in process 

 of formation. The mitotic figure is in the anaphase stage, and was found in a 

 germ cell free in the body cavity of a young daughter redia. These data on 

 the origin of the germ cells from the specialized germinal mass at the blind 

 end of the gut support the thesis of Leuckart (1886:123) and Schwarze (1886: 

 48, 49), that the cells have preserved their original embryonic character. The 

 fact that the production of the polar body and consequent maturation of the 

 germ cells takes place in cells next to the body cavity as well as in those free in 

 the body cavity, explains the observations of Thomas (1883:115) that some 

 of the cells from which the germ balls are derived are "the germinal cells of 

 the embryo or cells derived from them by division, others are formed by a 

 proliferation of the epitheHum lining the cavity of the sporocyst," since these 

 two groups are traceable back to a common origin at the base of the gut. 



A description is now given in support of the view that the germ cell is 

 a true ovum. In its unmodified condition the germ cell is moderately incon- 

 spicuous, similar in all respects to an undifferentiated parenchyma cell. As it 

 begins to change, the cell enlarges, the cytoplasm becomes granular, with 

 many interstitial vacuoles, and the nucleus comes to have a clearly outhned 

 membrane wall. Frequently the chromatin material is massed into a karyo- 

 some (Fig. 46 A). The chromatin mass now becomes oblong (B) and after 

 considerable growth becomes coiled into a thick skein (C). The next stage 

 (P) shows the division of the skein into eight chromosomes. These chromo- 

 somes arrange themselves in an equatorial plate, and soon show a longitudinal 

 sphtting. One of these (b) is precocious in its behaviour. It wanders toward 

 the edge of the nucleus and divides (6i, 62) while the other seven chromosomes 



