140 



THE THEORY OF THE GENE 



and if, as Oscar and Grunther Hertwig have shown, these 

 eggs are then fertilized, they may produce embryos whose 

 cells have the half number of chromosomes. Conversely, 

 if the spermatozoa of the frog are radiated they may 

 enter the eggs, but may fail to take further part in the 

 development. Under these circumstances the egg may 

 develop, for a time, with a haploid set derived from the 

 egg nucleus. In some of these eggs, on the other hand, 



Fig. 83. 



Egg of Triton constricted in two, immediately after fertilization. 



In the right half the polar body is shown. (After Spemann.) 



the chromosomes of the egg may first divide without the 

 protoplasm dividing, and in this way the full number of. 

 chromosomes is restored before development begins. 

 These eggs produce embryos that develop into normal 

 tadpoles. 



Most of the artificial haploid forms obtained in these 

 various ways are weak. They die, in most cases, long 

 before the adult stages are reached. It is not evident why 

 this should be true, but there are several possibilities 

 that may be taken into account. If a whole egg with a 

 haploid nucleus is incited by artificial means to partheno- 

 genetic development, and if, before differentiation sets 



