DEVELOPMENT OF P.\RACOPIDOSOMOPSIS 25 



tation comparatively easy. The polygerm is surrounded by the 

 polar membrane, from the inner surface of which processes ex- 

 tend in toward the center (fig. 52, P.M.). In more advanced 

 stages these processes are invaded by the polar nuclei, and the 

 membrane thus formed eventually surrounds each primary mass, 

 becoming what we shall call the outer envelope or membrane of 

 the mass. 



Another point of interest in this preparation is the condition 

 of the inner membrane nuclei. These nuclei and their accom- 

 panying cytoplasm are in the act of forming the inner envel- 

 ope of the primary masses. Both stain very lightly (fig. 52, 

 I.M.C.). Various stages in the formation of the inner envelope 

 are seen in the preparation. In the upper part of the figure 

 the nuclei lie free in the cytoplasm which surrounds the dark 

 embryonic cells. On the lower side of the formative primary 

 mass lying on the right, a portion of the membrane is fairly 

 well organized. 



This account of the development of the inner and outer en- 

 velopes of the primary embryonic mass differs somewhat from 

 that given in an earlier paper, from which I may quote: "About 

 seventy hours after oviposition, the nucleated membrane begins 

 to invade the embryonic cells by the formation of trabeculae, 

 which divide the embryonic cells into several groups, or primary 

 masses. During the formation of these masses, or very shortly 

 thereafter, the young polygerm elongates in the direction of 

 the long axis of the egg. In addition to the nucleated membrane, 

 each primary embryonic mass develops a second envelope, which 

 lies just inside the nucleated membrane. Apparently this inner 

 envelope is formed from the peripheral layer of cells of the em- 

 bryonic mass" (Patterson, '18, p. 365). As a matter of fact, 

 a more extensive study of a completed series of sections shows 

 that the initial steps in the formation of the inner envelope pre- 

 cedes the development of the outer membrane. 



The true embryonic cells stand out in sharp contrast to all 

 other structures in the polygerm (fig. 52, D.E.C.). They have 

 become spherical in shape. In the section four primary masses 

 are seen. Two of these are practically completed, while two are 



