DEVELOPMENT OF PARAVORTEX GEMELLIPARA 531 



neath the external epithelium and in the vas deferens, while 

 manj^ have already entered the seminal vesicle. Thus it is 

 evident that Paravortex is protandrous to the extent that sperma- 

 tozoa are produced before the female reproductive tract has 

 become differentiated, for no lumen has appeared farther back- 

 ward than the atrium. 



d. Vitellaria. In the section wherem the earliest anlagen of the 

 sex organs were described it was noted that several large cells 

 with an unusual quantity of darkly staining cytoplasm and con- 

 spicuous nuclei were observed to arise in the region of the gonads 

 and to migrate posteriorly between entoderm and ectoderm 

 (fig. 43). While these cells became distributed on all sides of 

 the latter, a marked accumulation occurs at the extreme posterior 

 end (fig. 44). Hallez was of the opinion that these cells in P. 

 cardii contributed largely to the formation of the intestine, but 

 also to the mesenchyme and vitellaria. There is no evidence 

 that they gi^^e rise in P. gemellipara to any other organ than the 

 vitellaria. These cells are identical in appearance with the 

 young cells which lie at the extreme tips of the vitelline strands 

 in the adult. Figure 44 shows at once the embryonic cells and, 

 just outside the capsule, several of the young vitelline cells in 

 the mother's tissue. 



By the time the young worm has attained the stage represented 

 in figure 47 strands of cells similar to those of the adult have grown 

 forward from the posterior mass; at the same time the scattered 

 cells of the same type have divided to form groups here and 

 there through the mesenchyme of the posterior region. At x 

 two of them are undergoing mitotic division; similar figures 

 have been observed at the tips of the adult vitellarial strands. 

 As yet veiy few have appeared in proximity with the ovaries. 

 Hence there seems to be no evidence that in P. gemellipara the 

 vitelline glands develop, even in part, from the hermaphroditic 

 gland as it appears at this stage ; these organs lack all connection 

 with any other densely stained tissue. In the reconstruction 

 (fig. 52) from an older worm, however, the vitellaria have formed 

 a contact with the posterior ends of the ovaries. The various 

 groups of yolk cells have anastomosed to form an extensive gland. 



JOURNAL OF MORPHOLOGY, VOL. 27, NO. 2 



