16 THE ORIGIN OF GYNANDROMORPHS. 



bryos in which a process similar to that just described took place. 

 The unfertilized eggs were stimulated to parthenogenetic develop- 

 ment by placing them in sea- water containing a little valerianic acid. 

 After a few minutes the eggs were returned to sea-water and sperm 

 added. The sperm-nucleus did not penetrate in many cases until 

 the egg nucleus had begun to divide and then, as in Boveri's case, it 

 often united with one of the daughter nuclei. In neither of the cases 

 is there any elimination of single chromosomes, but in a more general 

 sense the earlier group of paternal chromosomes was dislocated in 

 that it failed to reach its normal destination. 



The extremely important experiments that Baltzer made with sea- 

 urchin eggs resulted in demonstrable cases of elimination, but here of 

 whole undivided chromosomes. For instance, when the eggs of 

 Strongylocentrotus are fertilized with the sperm of Sphcerechinus, it 

 is found at the first division of the egg that while some of the chromo- 

 somes divide and the halves move to opposite poles, other chromo- 

 somes remain in place, or become scattered irregularly between the 

 two poles of the spindle. They appear later as irregular granules and 

 show signs of degeneration, ana although remnants of them may 

 persist for a while, they take no further part in the development. 

 The maternal egg-nucleus contained in this case 18 chromosomes 

 and likewise the paternal sperm-nucleus. Hence, after union and 

 division, 36 chromosomes should go to each pole of the segmentation 

 spindle if all divided. Baltzer found, however, only 21 chromosomes 

 at each pole, which means that 15 chromosomes have failed to behave 

 normally, and it is probable that these are derived from the paternal 

 nucleus. Three chromosomes only of the latter, on this interpreta- 

 tion, take part in the division. In consequence, the nuclei of the 

 embryo contain almost exclusively maternal chromosomes, and it 

 is significant that the larvae are largely or entirely maternal in char- 

 acter. It is true that we have no evidence to show at present that 

 the larvse of these sea-urchins differ in only one or more Mendelian 

 factors. It would be very surprising if such were the case, yet the 

 results show at least so great a preponderance of maternal characters 

 that we must infer that the three surviving paternal chromosomes 

 produce no marked difference. 



The reciprocal cross gave a different result. When the eggs of 

 Sphcerechinus are fertilized by the sperm of Strongylocentrotus, division 

 of all of the chromosomes takes place normally and 36 are found at 

 each pole. The pluteus that develops shows peculiarities of both 

 paternal and maternal types. The difference between the two crosses 

 is probably due to the observed differences in the behavior of the 

 chromosomes. In the first case, the lagging and subsequent degenera- 

 tion of certain chromosomes may be spoken of as a sort of elimination, 

 although the causes that bring it about must be supposed to be of a 



