52 



PHYSICAL BASIS OF HEEEDITY 



pear they show the identical irregularities (Fig, 22, a 

 to d) . It is probable, therefore, that each chromosome has 

 retained the particular form that it had when it passed into 

 the resting stage ; or at least that the axial thread from 

 which the network was spun out has remained in place. 



In a few cases the chromosomes appear more or less 

 visible during the resting stages. This, however, is such 

 a rare event that it is doubtful whether it can be appealed 

 to in support of the view that in other cases the chromo- 

 somes remain intact. 



a o c a 



Fig. 22. — Sister blastomeres of Ascaris preparatory to another division, showing similar 



arrangements of chromosomes. (After Boveri.) 



The most convincing evidence comes from exceptional 

 cases of accidental or irregular distribution of one or 

 more chromosomes, so that an ^gg^ or a cell comes to have 

 one more chromosome than is usually present. In the 

 thread-worm Ascaris there are two varieties — one that 

 has four chromosomes in the embryonic cells (with two as 

 the reduced number) and another variety with two 

 chromosomes (with one as the reduced number). A few 

 females have been found in which the unfertilized eggs 

 contain one of these numbers, and all of the spermatozoa 

 that have been received from another individual the 

 other number. In such cases the fertilized eggs, and 



