V ASCARIS 193 



of chromatin constituting the gonad and eventually developing into 

 functional gametes in the way already indicated. 



The phenomena that have been described are of much importance 

 in relation to the known facts of heredity. In the first place it may be 

 recalled how the study of the Protozoa brought out the fact that the 

 nuclear material of the cell plays an important part in governing and 

 controlling its vital activities. 



When we study the phenomena of inheritance in animals in general, 

 one of the most striking features of these phenomena is seen to be that, 

 on the whole, the two parents contribute in equal parts to the characters 

 of the offspring. It is clear in a large proportion of cases that the total 

 inheritance of the young individual is already contained within the 

 zygote, for this may go through the complete course of its development 

 into the new individual under conditions which render it impossible 

 that any moulding influence can be exercised by the parent. Conse- 

 quently the inheritance must have been brought into the zygote, and 

 brought in in equal parts, by the two gametes. Therefore the material 

 basis of inheritance, the special substance whatever it is that carries 

 inheritable qualities, must be contributed to the zygote in equal quan- 

 tities by the two gametes. These two gametes may of course be enor- 

 mously different in their size, for example in a bird the macrogamete 

 may be millions of times the size of the microgamete — but the evidence of 

 Ascaris, and also of many other animals, indicates that, in spite of 

 superficial differences in the size of the two gametes, there is one element 

 contributed in approximately equal parts by the two gametes, namely the 

 chromatin of the nucleus apart from the sex-chromosome. As there is 

 so far no evidence to prove that this holds for any other substance, 

 the presumption is obviously very strong that the chromatin is the 

 vehicle which carries the hereditary qualities. 



Another striking feature of heredity is the way in which it permeates 

 every portion of the body. In any part of the body, in any tissue, 

 a feature may make its appearance which has clearly been inherited from 

 one or other parent. But we have already seen how the ordinary chromo- 

 somes of the zygote, derived half of them from one parent and half from 

 the other, are at each one of the countless mitoses which take place during 

 the development of the individual accurately split into two halves, one 

 going to the one daughter cell and one to the other, so that the meta- 

 bolism of every resultant cell is controlled by chromosomes half of 

 paternal origin and half of maternal. Clearly this fits in perfectly with 

 the idea that the chromatin is the bearer of heredity. 



Still another point. One of the striking features of heredity is that 



O 



