DEVELOPMENT OF PARA VORTEX GEMELLIPARA 497 



The first polar body at the left of the ovum has approxmiately 

 twice the diameter of the second. That is it unmistakably 

 smaller than the first two micromeres can be determined by 

 comparing it with the first cleavage figures seen in text figures 

 10 and 11, while in text figure 12, drawn from life, the polar bodies 

 and first micromere can be directly compared. 



c. FertilizatiGn. As stated above, insemination occurs before 

 the onset of maturation. The point of entrance, illustrated 

 by Patterson's figures and by text figures 6 and 9 of this paper, 

 is somewhere in the hemisphere opposed to that from which 

 the polar bodies are extruded. During maturation the sperm 

 nucleus, after losing its flagellum, moves toward the center of 

 the egg, at the same time enlarging to form the irregular male 

 pronucleus. 



Before their union both pronuclei become curiously amoeboid 

 or lobular in form (text fig. 7). Mattieson ('04) noted this 

 phenomenon in Planaria torva and P. polychroa, and Bresslau 

 ('04) observed it in several Rhabdocoeles. The same was ob- 

 served by Hallez ('09) in P. cardii. 



The male pronucleus in text figure 6, having entered at some 

 point near the vegetal pole, has penetrated the egg for about 

 one quarter of its diameter. A distinct darkly stained mass 

 of chi'omatin occupies the center of the otherwise chromatin- 

 less vesicle. Since, unfortunately, this preparation was rather 

 heavily stained it is impossible to discern yv^ith certainty any 

 degenerating remains of the sperm flagellum among the count- 

 Text fig. 7 Egg showing the lobulated male and female pronuclei. X 730. 



Text fig. 8 Thick section of an egg containing the first polar spindle. One 

 centrosphere has divided and the other is in the process of division, cf male 

 pronucleus. X 730. 



Text fig. 9 Section of an egg from the same capsule as the preceding. The 

 first polar body is just being given off. sp., spermatozoon; mit., mitochondrial 

 mass; p.bl. first polar body. X 730. 



Text fig. 10 Reconstruction from sections of a pair of embryos in both of 

 which the first micromere has been given off. mad. first macromere; micl. 

 first micromere. X 730. • . 



Text fig. 11 A pair of embryos from one capsule, each consisting of two 

 macromeres and two micromeres. mad, mac.^, first and second macromeres; 

 mid, mic.2, first and second micromeres. X 730. 



