34 HOW ANIMALS DEVELOP 



there will be two normal genes giving normal adults. 

 Hereditary factors of this kind were discovered by 

 Mendel in the middle of the last century, and he 

 also gave rules for the way in which they are 

 inherited. Chromosomes had not been described at 

 that time, and it is only about thirty years since it 

 was realized that the hereditary factors actually lie 

 on the chromosomes and that Mendel's laws are 

 perfectly well explained by the behaviour of the 

 chromosomes which we have described above. The 

 theories propounded by Mendel are collectively 

 known as '^ MendelisnC and are part of the science 

 of genetics^ or the study of heredity. 



It is clear, then, that the chromosomes, or the 

 genes within them, play a leading role in develop- 

 ment, and we shall have to discuss later (see 

 Chapter vii) how they do it. But we can say now 

 that an ^gg can develop without a full double set 

 of chromosomes ; it can develop quite well, so long 

 as it has got a half or haploid set. Anything less than 

 this is fatal, and so usually is anything between a 

 half and a whole set because of its lack of balance. 

 Professor Dalcq in Brussels is investigating the 

 development of frogs' eggs with less than the haploid 

 number of chromosomes, and is finding out just 

 when and how the embryos fail. 



The other process in fertilization is the activation 

 of the tgg. We know very little about how this 

 happens. What it does is to cause the ^gg to start 

 dividing and developing. Now the same change can 

 be brought about by other things which are not the 



