DEVELOPMENT OF PARA VORTEX GEMELLIPARA 469 



about twenty-five spermatids at a stage so yoimg that the 

 differentiation of the flagelhmi had but just begun. The nu- 

 clei could readily be seen in those cells which lay near the 

 surface. Numerous spermatozoa were present as well. On 

 another occasion a livmg animal while mider observation on 

 a slide became crushed. Among the mesenchyme cells which 

 escaped from the body were many spermatids. Each cell 

 had the form of a thick concavo-convex lens attached to the 

 flagellum at such an angle as to give the spermatid the appear- 

 ance of a ladle. These spermatids are always present in the 

 seminal vesicles of worms whose ovaries have begun to set 

 eggs free, but m the very young in which the testes have just 

 commenced to fmiction all the sperm cells are mature on leaving 

 the testis. 



Female reproductive organs 



As the male organs degenerate those of the female apparatus 

 undergo rapid development. By the time the animal is two- 

 thirds grown the ovaries and vitellaria are prominent. The 

 ovaries are situated one on either side of the body below the 

 median horizontal plane. At the small anterior end of each 

 ovary, which lies just posterior and slightly dorsal to the testis 

 of the same side, numerous oogonia are crowded irregularly 

 together. Cell membranes can be distinguished except between 

 the first two rows of nuclei. In some specimens a terminal 

 filament passes forward from the tip of the ovary and joins an 

 indistinct mass of cells on the posterior ventral surface of the 

 testis. As will be seen in the part of this paper which treats 

 the development of the reproductive glands, the ovary and 

 testis arise from a conmion mass of undifferentiated cells, and 

 later usually become separated. Where in later stages this 

 strand of cells remains between the two glands it is regarded 

 as a bridge which has failed to break down. 



Strikmg spireme figures are to be observed in the oogonia. 

 A short distance back from the anterior end of the ovary, how- 

 ever, only cells with resting nuclei occur. These are primary 

 oocytes. Beyond this point the flattened cells are packed 



