34 BULLETIN OF THE 



glands increase in size, but do not grow as rapidly as the surrounding 

 protoplasm, and therefore the nucleus becomes smaller in proportion to 

 the size of the cells. Many highly refractive granules appear in the 

 protoplasm, and increase in number with the growth of the cells, till 

 eventually, when the cells have attained their full size, they form a 

 relatively large proportion of the cell mass (Plate IV. Fig. 45). Cor- 

 responding to the changes that take place in the yolk cells, there is a 

 slight increase in the size of the parovarial cells, in which there is also 

 an accumulation of highly refractive granules (Plate IV. Fig. 44), but 

 the nuclei retain more nearly their original proportions to the cells 

 than in the case of the yolk cells. In addition to the identity of 

 histological structure, a most striking evidence of the derivation of the 

 yolk glands from the parovarial cell masses is found in the reaction of 

 both kinds of cells with staining fluids, more especially with picrocar- 

 minate of lithium. Figures 44 and 45, Plate IV., represent respectively 

 sections through parovarial cells and mature yolk-gland cells of the same 

 individual. Figures 19 and 22, Plate II., are sections from another 

 individual ; Figure 22 is a section of young parovarial cells, and Fig- 

 ure 19 of incipient yolk -gland cells. Upon comparison of Figures 19 

 and 22 with Figures 45 and 44, it will be noticed that, in addition to 

 the appearance of the granules in the protoplasm of the older cells, 

 there has been an increase in the size both of the yolk cells and of the 

 cells of the parovarium. It is my belief, then, that the two large 

 dendritic yolk glands arise by cell proliferation from the parovarial 

 organs which exist in intimate relation with the ovaries. 



lijima ('84, p. 412) describes a pair of structures lying in front of the 

 ovaries in Polycelis tenuis as being composed of a solid mass of cells, 

 and as i*esembling young ovaries, so that this species possesses, in his 

 opinion, two pairs of ovaries, one of which is rudimentary. - On account 

 of their terminal position, he considers these rudimentary structures, 

 although not the functional ovaries, as the homologues of the single 

 pair of ovaries present in other species. His account of the growth of 

 the yolk glands, as given at p. 417, coincides with my observations, but 

 concerning the source of the chains of young yolk cells he says (p. 455) : 

 " Wir diirfen sagen, dass die Dotterstriinge durch Vermehrung einzelner 

 Zellen, welche in dem Mesenchym sich befinden, ihren Ursprung nehmen." 

 But his evidence that the " Dotterstrange " arise in situ is not satisfac- 

 tory. It is to be regretted that he has not given a fuller account of the 

 so called rudimentary ovaries of Polycelis, which, I am almost certain, 

 are the equivalents of the parovarial cell masses of Phagocata. 



