MUSEUM OF COMPARATIVE ZOOLOGY. 33 



Intimately associated with the ovaries are two prominent compact 

 cell masses with deeply stained nuclei, which may provisionally be called 

 parovaria (Plate II. Fig. 21, vCm.). They are larger than the ovaries, 

 and envelop them above, in front, and on the outside ; that is to say, 

 the ovaries are surrounded on three sides, being partially imbedded, so 

 to speak, in these cell masses. The latter are present in every indi- 

 vidual, and their size relative to that of the ovaries varies only with the 

 condition of the sexual organs. They are smallest during the develop- 

 ment of the spermatozoa, and are most prominent at the time when the 

 yolk glands have reached their full development. For a long time these 

 cell masses puzzled me. I believed them to correspond to the second 

 pair of rudimentary ovaries described by lijima ('84, p. 412) for Poly- 

 celis tenuis, and I at first accepted his interpretation of their significance; 

 but sections through additional material, where the female organs were 

 not so advanced, served to show their true meaning ; they are the organs 

 which give rise to the yolk glands. At an early stage in the development 

 of the testes no yolk glands are present, but they begin to appear at the 

 time when the spermatozoa are ripening. 



The first traces of the yolk glands are seen in branching chains of cells, 

 which arise as outgrowths from the parovaria. Each cell has a large 

 nucleus that is stained deeply in carmine. In these chains the cells lie 

 either in a single row, or it may be in several rows (Plate II. Figs. 19 

 and 19«). The nuclei are large and granidar, and occupy the greater 

 part of the cell. It is to be inferred that the cells are dividing rapidly, 

 since nuclei are found in all stages of division, and two nuclei are 

 frequently seen in the same cell ; the division appears to be direct, or 

 amitotic (Plate II. Figs. 19 and 22). The rudimentary yolk glands oc- 

 cupy at first the ventral regions around the oviducts, but afterwards 

 they send branches from there dorsad, until there is formed a dendritic 

 system of rapidly dividing cells, w-hich ramify through the tissues. 

 From each of the cell masses around the ovaries is derived one half 

 of the yolk system, that belonging to its own side of the body. The 

 cell chains of the young yolk glands are seen to be directly connected 

 with the parovarial cell masses, and histologically the structure of the 

 two is identical (compare Fig. 19 with Fig. 22, Plate II.). Furthermore, 

 at the time of development of the yolk glands there is a very active 

 division of the cells of the parovarial masses, a condition that does not 

 exist when the yolk glands have matured. A similarity in the condition 

 of the cells of the yolk glands and those of the parovarial masses is 

 evident at all stages of development. The young cells of the yolk 



VOL XXI — NO. 1. 3 



