17] LIFE HISTORY OF TREMATODES— FAUST 17 



remain with their halves still in contact. The precocious chromosomes take 

 up positions toward the poles of the cell (G). The other chromosomes then 

 divide and migrate to opposite poles {H), one of these daughter groups being 

 constricted off as a polar body (/). As a result of this process eight chromo- 

 somes separate by longitudinal splitting, so that half of each goes into the polar 

 body and half remains in the cell. The polar-body remains in cytoplasmic 

 connection with the ovum while the latter undergoes another division. As in 

 the previous division, simple mitosis occurs. The chromosomes ^i, 62 precede 

 the others in separation into component halves {{Fig. 46 /). In a late anaphase 

 of this second division (/) the polar body may divide, altho this is not always 

 the case. 



This second division is not a part of the maturation, for that has been 

 accomplished by the expulsion of the single polar body: hence, it constitutes 

 the first division of the mature ovum. After this {K) the polar body is entirely 

 separated from the blastomeres (L) and disintegrates. Thus maturation 

 consists of a single mitotic division with the extrusion of a polar body, and 

 takes place without any reduction of chromosomes. In other words, the 

 process is one of true parthenogenesis. 



The somatic chromosome count of the developing germ ball is eight, con- 

 sisting of seven ordinary chromosomes and the precocious individual. In 

 support of this statement is the count of each of the first two blastomeres 

 (JfiT, L), and the chromosome complex in the late metaphase of an endoderm 

 cell of a morula {M). In the latter the count is double, e.g., sixteen, in view 

 of the previous spHtting of the chromosomes antecedent to separation into the 

 daughter chromosome groups. The consistent tendency of the chromosome 

 b and its descendents to separate from the chromosome mass and to divide 

 before the other seven split, suggests the possibility that this chromosome is a 

 heterosome, two of which Lindner (1914) has found in the adults of Schistosoma 

 haematobium. 



In the case of the germ balls that never reach the body wall, the process 

 of maturation takes place free in the body cavity. For those cells which 

 lodge against the wall and even fuse with the wall, the process of maturation 

 and cleavage into two blastomeres takes place while the ovum is still in con- 

 tact with the body wall. At this time it is set free and allowed to develop into 

 a germ ball. 



In the older mature rediae (Fig. 44) the epithelial layer of the body wall 

 Hning the body cavity consists of a syncytium in which nuclei are arranged 

 irregularly. The cell boundaries become distinct only as maturation of the 

 cells approaches. 



Leuckart (1886:124) has stated that it is relatively long after germ ball 

 formation before it is evident whether the embryo is to develop into a redia or 

 cercaria. While the chromosomal history in the rediae of Cercariaflabelliformis 

 shows no difference between cells which develop into daughter rediae or cer- 

 cariae, the cytoplasmic history of this species is indicative of the generation 



