BANCROFT: OVOGENESIS IN DISTAPLIA OCCIDENTALIS. 65 



by the splitting of the genital strand, which is progressively converted 

 from behind forward into vas deferens and oviduct. But with the 

 growth of the bud, the undifferentiated portion of the strand is stretched 

 more and more, until it becomes a single row of cells attached end to 

 end, which connects the last differentiated portions of the two ducts 

 with the peribranchial epithelium. 



From this time on cell-multiplication must take place pari passu 

 with the separation of the ducts. But as no mitoses were encountered, 

 it is useless to speculate as to whether this multiplication is accom- 

 plished in the region of the genital strand, the differentiated ducts, or 

 the mass of cells where the three join. Even before the ducts are 

 separated throughout their whole length a histological difference is 

 apparent, the nuclei of the oviduct cells being more flattened and tak- 

 ing a fainter stain than those of the vas deferens. Another point of 

 interest is that, contrary to what obtains in other species, both ducts 

 are solid strands, which do not acquire a lumen until much later. 



While the ducts are being separated, the testis increases in size and 

 entirely covers the deep side of the ovary, so that in cross-section it 

 appears as a crescent, the tips of which abut against the ectoderm on 

 either side of the ovary. Later it becomes divided up into the eight 

 or more testicular pouches which are characteristic for the species. 

 Meantime the ovary increases in size and acquires a lumen, on the 

 deep side of which most of the ova are situated. With these last 

 changes all the essentials of the adult structure are reached, but the 

 position of the sexual organs with respect to the axes of the animal, and 

 the point of union of the genital strand with the peribranchial sacs, 

 still undergo some change. 



In young buds with small peribranchial sacs, which have not yet 

 united to form the median cloaca, and without a trace of stigmata, the 

 sexual organs are situated in the mid-dorsal line, and also in the 

 intestinal loop which has developed around them. At this stage the 

 oesophagus opens into the right posterior corner of the branchial sac 

 and the anus is near the left posterior corner, so that the whole of the 

 post-pharyngeal digestive tract is in a frontal plane. The genital 

 strand extends forward in the median line and joins the tip of the left 

 peribranchial sac, where this comes nearest to the sagittal plane. Later 

 the point of union of the genital strand with the peribranchial sac 

 migrates still farther to the left, so that it comes to lie much nearer to 

 the anus. Now, as the bud grows and assumes the adult shape, the 

 whole abdomen is so twisted that right becomes ventral, left dorsal, 



