39 

 The female pronucleus Is now formed and proceeds to the 

 center of the egg where It meets the male pronucleus. Figures 

 116 and 116 are drawn from nuclei fixed at 2 hours and 15 min- 

 utes after spawning. At this time it is not possible to tell 

 which is the male and which the female pronucleus tmless the 

 slightly concave side of the one shown in Pig. 116 indicates 

 that it is related to the nucleus shown in Fig. 112. In this 

 case it would he the male pronucleus. The nuclei have grown 

 rapidly and continue to do so until they reach the center of 

 the ogg. Their contents are finely granular. These granules 

 increase in size as the nuclei hecome larger. Figures 118 and 

 119 show the position of the nuclei four hours after spawning 

 and Fig. 120 (from en agg six hours old) shows them lying side 

 by side in the center of the egg. They have become elongated 

 and many times larger than they were when first formed. From 

 the above description of fertilization it is evident that the 

 nuclear cup takes no part in the formation of the male pro- 

 nucleus for the latter is derived from the sperm-vesicle which 

 is the Inverted capsule. 



This completes the description of the structure and be- 

 havior of the male cells in the stone-crabe. We have here some- 

 thing unique in the method by which the sperm enters the egg 

 and something exceptional in the phenomena of fertilization. 

 These observations raise several theoretical questions, some 

 of which we will now briefly consider. 



IX. Discussion. 

 We have here a case in which an Infolded vacuole that 

 arose in the cytoplasm is everted through the shell of the 

 egg and fertilizes it. How may such an event be brought into 



