ON PIEREDITY. 545 



acteristics, yet, as every one knows, the child is uot absolutely like its 

 parents, but possesses its owu character, its own individuality. It is 

 easy for any one to recognize that differences exist amongst men when 

 he compares one individual with another; but it is equally easy for 

 those who make a special study of animals to recognize individual dif- 

 ferences in them also. Thus a pigeon or canary fancier distinguishes 

 without fail the various birds in his tlock and a shepherd knows every 

 sheep under his charge. But the anatomist tells us that these differ- 

 ences are more than superticial, — that they also pervade the internal 

 structure of the body. Intimately associated thereforfe with the con- 

 ceittion of heredit;^' — that is, the transmission of characters common to 

 both parent and oftspring — is that of variability, — that is, the appear- 

 ance in an organism of certain characters which ^re unlike those ])os- 

 sessed by its parents. Ileredity therefore may be defined as the per- 

 petuation of the like ; variability, as the production of the unlike. 



And now we may ask. Is it possible to ofler any feasible explanation of 

 the mode in which variations in organic structure take their rise in the 

 course of development of an individual organism ? Anything that one 

 may say on this head is of course a matter of speculation, but certain facts 

 may be adduced as otiering a basis for the construction of an hypothesis, 

 and on this matter Professor Weisraaun makes a number of ingenious 

 suggestions. 



Prior to the conjugation of the male and female pronuclei to form the 

 segmentation nucleus, a portion of the germ-plasm is extruded from the 

 egg to form what are called polar bodies. Various theories have been 

 advanced to account for the significance of this curious i)henomenon. 

 Weisman explains it on the hyi)othesis that a reduction of the number of 

 ancestral germ-plasms in the nucleus of the egg is a necessary prepara- 

 tion for fertilization and for the development of the young animal. He 

 supposes that by the expulsion of the ])olar bodiesone-half of the num- 

 ber of ancestral germ-plasms is removed, and that the original bulk is 

 restored by the addition of the male pronucleus to that which remains. 

 As precisely corresponding molecules of this plasm need not be expelled 

 from each ovum, similar ancestral plasms are not retained in each case; 

 so that diversities would arise even in the same generation and between 

 the oftspring of the same parents. 



Minute though the segmentation nucleus is, yet microscopic re- 

 search has shown that it is not a homogeneous structureless body, 

 but is built up of diffierent parts. Most noteworthy is the presence 

 of extremely delicate threads or fibrils, called the chromatin filaments, 

 which are either coiled on each other, or intersect to form a net- 

 work-like arrangement. In the meshes of this network a viscous — and, 

 so far as we yet know, structureless — substance is situated. Before the 

 process of division begins in the segmentation nucleus these filaments 

 swell uj) and then ])roceed to arranges themselves at first into one and then 

 into two star-like figures before the actual division of the nucleus takes 

 n. Mis. 224 35 



