28 The Sexual Organization of the Rhizocephala. 



In figures 7 a and 8 are given two drawings of some eggs which were extracted from 

 the mantle-cavity at this stage and kept for a quarter of an hour in sea water. The result 

 was taht the second polar spindle was broken down and the chromosomes scattered so as to 

 make them much easier to count. Fig. 7 a is a whole view showing the first polar body 

 (j)bl) and 14 single chromosomes; figs. 8 and 8 a represent sections showing apparently 12 

 chromosomes many of which have the appearance of dyads. 



The second spindle may often travel some distance away from the first polar body as 

 shown in fig. 9 Plate 3. 



The telophase of the second spindle is shown in fig. 9 a, and the formation of the second 

 polar body {pb2) is indicated. 



By this time the first polar body (pbl) has been extruded far out of the egg and is 

 seen upon the membrane which surrounds the egg at this period. The exact nature of this 

 membrane I cannot explain but it appears to be formed at the time of fertilization. At this 

 point fertilization occurs, and the entrance of the spermatozoon is the signal for a change in 

 the distribution of cytoplasm and yolk in the egg. Hitherto there has been no concentration 

 of cytoplasm at any particular region, but now a wedge of cytoplasm is formed immediately 

 beneath the point where the polar bodies have been extruded and including the newly entered 

 spermatozoon and the egg pronucleus (fig. 10). 



The male and female pronucleus are at first clearly distinguishable from one another 

 as shown in figure 10. In the male pronucleus we can still distinguish the immensely elon- 

 gated chromatic strand of the spermatozoon, but this rapidly breaks up into small granules, 

 the nuclear membrane becomes more spherical, and the chromatin becomes finally agglomerated 

 into a nucleolus, and the male and female pronuclei are indistinguishable in appearance. 

 Neither in the recently entered spermatozoon nor in the male pronucleus can a trace of 

 centrosome or sphere be as yet distinguished. 



The cytoplasmic wedge with the two pronuclei now passes to the centre of the egg 

 fig. 11} while on the periphery we can recognize the second polar body [pb2) brightly staining 

 and lying within the egg membrane, and in favourable cases the first polar body {pbl) lying 

 outside the egg membrane and rapidly fading from view. After this stage the first polar body 

 is no longer visible in preparations, but the second polar body can be easily seen even after 

 segmentation has begun. The fact of the rapid disappearance of the first polar body for a 

 long time deceived me into suspecting parthenogenesis, and the difficulty of getting the fertili- 

 zation stages, which are so rapidly passed through, confirmed me in this error. 



As the pronuclei pass to the centre of the egg they increase greatly in size and their 

 chromatin becomes more diffuse, and now they proceed rapidly to pass again to the periphery 

 of the egg together with the cytoplasm which surrounds them (figs. 12 and 13). 



They always arrive at the periphery at the same point from which they started namely 

 immediately beneath the second polar body. In figures 14 and 15 the pronuclei are shown 

 after regaining the periphery and increasing greatly in size. 



