The Role of the Chromosomes 203 



taught tlie physical and natural sciences in the monastery 

 school. While monk and teacher he was essentially a great 

 investigator, and in spite of his other duties, he found time 

 to perform a large number of breeding experiments with 

 sweet peas, the results of which he published in 1865 in the 

 ' * Proceedings of the Natural History Society of Briinn. ' ' This 

 paper, which he sent to his friend Niigeli the botanist, made 

 no impression on the latter and attracted no attention, until 

 thirty-five years later, when Mendel's principle was inde- 

 pendently discovered by three botanists — DeVries, Correns 

 and Tschermak. Since then, Mendel's discovery has been 

 recognized as one of the greatest in biology, and his paper 

 has become a great scientific classic. 



The results of his work have been so extensively quoted, 

 and are known so widely and so well that their rehearsal is 

 needless here. There are certain features of his results how- 

 ever which, while well known to biologists, are perhaps not 

 fully appreciated by the general reader, and which it may 

 therefore be worth while to emphasize. Thus it is commonly 

 known, for example, that a cross between a tall pea and a 

 dwarf produces only tall offspring, which, when bred to- 

 gether, produce, on the average, three tall and one dwarf 

 descendants. But the meaning of this well-known Mendelian 

 ratio is possibly not widely understood. A ready explanation 

 is found however in the behavior of the chromosomes of the 

 germ cells, prior to, and during fertilization. 



The nucleus of an undividing or resting cell contains a sub- 

 stance known as chromatin, which, when the cell is sectioned 

 and stained for microscopic study, appears as a mass of deeply 

 stained blotches and specks scattered indiscriminately over 

 a very delicate network of threads or "linin" fibrils. When 

 the cell becomes active and starts to divide this chromatin 

 material is gathered together into an irregular twisted thread 

 known as the "skein" or "spireme," which is at first long 

 and thin, but soon shortens and thickens and then breaks up 

 into a number of segments in the form of rods, loops or 

 balls, the number of which is characteristic for any given 

 species of plant or animal. ])ut which varies in different 

 species from two to upwards of two hundred. In division 

 these chromosomes are equally divided so that each new cell 

 receives the same number as the parent cell contains. 



But when the animal or plant is ready to reproduce there 

 is a striking difference in the behavior of the chromosomes— 

 a difference to which is probably related all the varied and 

 wonderful phenomena displayed by Mendelian inheritance. 



Nearly forty years ago Van Beneden ascertained that the 

 germ cells of Ascaris, an intestinal parasite of the horse, each 

 contained, at the time of fertilization, one-half the number 



