530 STANLEY C. BALL 



organs are also being differentiated. It is evident that the 

 seminal vesicle arises from the anterior portion of the mass; 

 only a small number of cells at the extreme anterior end go to 

 form the vasa deferentia and the external epithelium of the 

 testis. 



In the region ventral and posterior to the seminal vesicle the 

 atrium has differentiated simultaneously. Like those of the 

 former, its walls are composed of two layers of cells, an inner 

 cubical epithelium and an outer sheet of flattened cells. As yet 

 no connection of its lumen with the outside of the body has been 

 effected. Indeed, the cells are not even in close contact with 

 the body wall. 



From the posterior surface of the atrium a dense mass of cells 

 leads a short distance dorsally. The antrum femininum will 

 arise later in this position. On the ventral surface of the mass 

 is a group of cells whose rounded posterior ends have begun to 

 reach backward behind the atrium. These cells enlarge to 

 form the single-ceUed shell-glands. 



Of the oviducts nothing is as yet present but a loose string of 

 undifferentiated cells which lead from the antrum anlage pos- 

 teriorly and dorsally to the ventral surface of the ovary. It was 

 impossible to distinguish about the latter such an epithehum as 

 was described for the testis, nor is it evident in later stages. 



The ovaries in the worm from which figure 50 was drawn are 

 well advanced. The older oocytes have undergone a consider- 

 able growth. 



Figure 51, from a somewhat larger worm, is presented to show 

 clearly the strand-like structure of the testis and the point at 

 which the vas. deferens enters the seminal vesicle. The oogonia 

 occupy the anterior region of the testis. These, by growth, 

 give rise to the large primary spermatocytes lying on the ventral 

 and dorsal sides; while in the center of the posterior half a row of 

 smaller, densely stained cells are regarded as secondary sper- 

 matocytes. No spermatids were distinguished with certainty, 

 but mature spermatozoa are conspicuous in the anterior central 

 region whence they can be traced dorsally between the cell 

 strands to the upper surface of the testis. Others lie just be- 



