R. C. VON BORSTEL 183 



past metaphase I in eggs still in the uterine sacs, a condition 

 never observed in females from other stocks. Meiosis in these 

 cases was observed to be continuing in the interior of the anterior 

 portion of the egg, w^here the blocked metaphase nucleus usually 

 lies. The phenomenon of uninhibited meiosis would seem to be as 

 rare an occurrence as mosaic production. Since in our study only 

 those females that show uninhibited meiosis also produce mo- 

 saics, and vice versa, it seems justifiable to correlate continuation 

 of meiosis in the egg interior with mosaic production and to con- 

 clude that the faulty meiotic block is in some way responsible 

 for induction of mosaics. In agreement with this conclusion, P. W. 

 Whiting (1932) found a wasp that was mosaic for reciprocal 

 crossover types, indicating two pronuclei from the products of a 

 single meiosis. It is conceivable that, if meiosis does not occur 

 at the egg surface, cleavage nuclei of two meiotic nuclei or more 

 (P. W. Whiting, 1934) might survive destruction. Furthermore, 

 this indicates that in normal eggs some agent present in the corti- 

 cal cytoplasm must be responsible for disintegration of the most 

 peripheral polar nucleus and inhibition of further division of the 

 fusion nucleus. The chromosome set that will become the pro- 

 nucleus pushes farther into the interior at each meiotic division 

 and escapes destruction. The simplest explanation then is that, 

 in mosaics, two or more meiotic nuclei must have escaped de- 

 struction by some agent in the egg cortex. 



That the egg cytoplasm can be incompatible to foreign nuclei 

 has been shown by the interesting cytological analysis of 

 Tchou-Su (1931). Working with amphibian eggs, he has shown 

 that foreign sperm can be destroyed by cytoplasm in a number 

 of different ways before fusion of the egg pronucleus and the 

 foreign sperm nucleus can take place. Nanney (1953), by cen- 

 trifuging Tetmhijmena undergoing conjugation, was able to dem- 

 onstrate that the position of the meiotic products in the cytoplasm 

 determined the later behavior of the nuclei. The cytoplasm in 

 this case certainly has a critical role in directing nuclear events. 



The most famous mosaics are the Eugster gynandromorph 

 honey bees, which were discovered in the middle of the nine- 

 teenth century and were puzzled over for the next seventy years. 



