DEVELOPMENT OF PARACOPIDOSOMOPSIS 13 



eggs, and there can be no doubt as to the manner in which the 

 polar nucleus is organized in this species. However, it is only- 

 fair to state that occasionally one finds eggs in which all three 

 polar bodies would appear to fuse, or at least only a single group 

 of chromatin thi'eads or rods can be detected (figs. 16, 24). 

 This appearance is perhaps more apparent than real and is 

 probably due to one of two causes. Either the polar body, 

 Ai, has akeady disintegrated or else it is hidden beneath the 

 formmg polar nucleus. 



The real proof that only two polar bodies enter into the pro- 

 duction of the polar nuclei is seen in certain eggs in which the 

 disintegration of the third polar body chromatin has been delayed 

 until the nuclear membrane is completely formed. Such a con- 

 dition is shoviTi in figures 22 and 36. 



Another line of evidence which supports our conclusion is 

 obtained in studying the mitotic figures of the dividing polar 

 nuclei. I have shown above that each polar body received 

 eight chromosomes. Therefore, if three polar bodies enter the 

 polar nucleus, its subsequent divisions should reveal twenty- 

 four chromosomes, or the triploid number. I have succeeded in 

 finding thi'ee clear metaphase plates, and in each case the diploid 

 number of sixteen chromosomes is present (fig. 25). 



c. Fertilization 



The egg is inseminated by a single sperm, which penetrates 

 the surface at any point on the posterior half. Polyspermy 

 never occurs. The entire spermatozoon enters (figs. 1, 3, 5, 

 S), but the tail disappears and only the head is transformed into 

 the male pronucleus. After maturation is completed, the ootid 

 group of chromatin forms the female pronucleus and at the same 

 time moves toward the sperm, which now lies at the posterior 

 end. The two pronuclei thus come to He close together (fig. 6). 

 Both nuclei then expand, come in contact with each other (fig. 7), 

 and finally fuse (fig. 8) to form a single large conjugated or 

 cleavage nucleus (fig. 9, F.N.), which can always be distin- 

 guished from the smaller cleavage nucleus of the unfertilized egg 

 (fig. 10, C.N.). 



