40 



POPULAR SCIENCE MONTHLY. 



The eggs of a sea urchin, Sphcerechinus, which normally develop 

 into a well-known larva, were broken by Boveri into two in such a way 

 that the nucleus was all contained in one fragment. Male cells of 

 another species of sea urchin, Echinus, having a well-known, but quite 

 different, larva, he then caused to enter the fragment without a nucleus. 

 A larva developed which possessed all the characters of the larva normal 

 to the male used in the experiment. Since cell contents of both species 

 were present and nuclear structures of only one, and the larva resembled 

 the species represented by a nucleus, it was concluded that the hered- 

 itary substance is located in the nucleus. The nucleus, then, contains 

 the physical basis of heredity. 



cv 



A 







* 



L 



Fig. 10. a. The Early Stage in the Maturation of the Egg. The four chromosomes 

 have been reduced to two tetrads. Eventually three out of each group of four granules will be 

 eliminated from the egg. b. The elimination of two of the granules of each tetrad in the for 

 mation of the first polar body. 



Boverr's results were looked upon with much suspicion until they 

 were confirmed by Delage. With a German and a Frenchman agreeing 

 we may safely consider this point as settled. 



The intimate process of the preparation of the hereditary cells for 

 their union and the union of the cells have been the subjects of many 

 monographs during the last twenty years. 



Since each cell has a definite number of chromosomes and this 

 number would be doubled by the union of two cells elaborate provisions 

 are made by the cells to reduce this number to one half before the union 

 of the two cells takes place. The study of the methods of this reduction 

 has engaged a host of cytologists during the past ten years, and innu- 

 merable papers have resulted. 



It is evident that the great difference between the two cells is simply 

 an adaptation to insure their union, for, after uniting, their nuclei, the 

 physical basis of heredity, become alike (c in figure 11). 



Credit for the great activity in research along this line must be 



