DEVELOPMENT OF PARA VORTEX GEMELLIPARA 529 



Bresslau ('04) also derived the mesenchyme from the undiffer- 

 entiated cells. 



The origin of this tissue in P. gemellipara agrees with that 

 described by these investigators except that there is no evidence 

 of a contributory action on the part of the large posterior cells; 

 it is all derived from the undifferentiated cell mass, from which 

 cells migrate posteriorly between the intestine and body wall. 

 The mesenchyme of the free-swiroming young worm from which 

 figure 47 was drawn already existed in the condition which is 

 characteristic of the adult. The cells are large and loosely 

 arranged with indistinct and irregular boundaries. 



c. Ovaries and testes. The youngest stage in the development 

 of the sex glands of P. gemellipara in which they can be recog- 

 nized is shown in figure 47. Although the testis lies close to the 

 anterior end of the ovary, the two glands are already separate. 

 No epithelium, however, has been formed about them. Since 

 this specimen had been stained with Ehrlich's haematoxylin, 

 and since the section was so thick that the entire testis and nearly 

 all of the ovary are included, the minute condition of the chro- 

 matin cannot be determined. The small dimensions of the 

 nuclei in comparison with those observed in the adult ovary indi- 

 cate that all the cells in the female gonad, except possibly the 

 most posterior, are oogonia. 



An important feature in this specimen is that the vitellaria 

 have become conspicuous, particularly in the posterior half of 

 the body. No connection between these glands and the ovaries 

 can be traced in this series oi sections. 



The next instructive stage which my material presented is 

 shown in figure 50, a sagittal section through one testis, the semi- 

 nal vesicle, atrium, shell-gland and the tissue which is to form 

 the antrum. In the testis the spermatogonia and spermatocytes 

 can easily be distinguished. Hallez described and figured a 

 similar appearance for the young testis of P. cardii. One feature 

 of which, however, he failed to give an account is that the gland 

 becomes provided with a flat epithelium. The latter is derived 

 from the mass of cells lying below the mid-ventral side of the 

 intestine. From this same mass the accessory reproductive 



