4 THE PKINCIPLES OF HEREDITY 



they are equally bearers of heredity. 1 After the conjugation 

 of the ovum and the sperm the resulting cell divides and re- 

 divides many times without conjugation occurring amongst 

 the descendant cells. These, like infusorians, if they do not 

 conjugate, ultimately die out. Most of them (the somatic or 

 body cells) are incapable of conjugation ; whilst such of them 

 as are capable of conjugation (the germ-cells) conjugate only 

 with cells from another body (i. e. cell family). There are, as 

 is well known, exceptions to this rule. Apparently unending 

 multiplication of cells may occur without conjugation, as 

 among such plants as are propagated by buds, slips, and 

 suckers ; and self-fertilization also occurs ; but the general 

 rule is as stated. A multicellular plant or animal, in the suc- 

 cessive stages of its development, is to be regarded, therefore, 

 as the homologue, not of the remote ancestral organism, 

 the homologue of which is the ovum or the sperm, but of 

 all those successive generations of unicellular organisms 

 which intervene between one act of conjugation and the 

 next. 



8. On the other hand, the cell-descendants of a pair of con- 

 jugated germs differ from the cell-descendants of a pair of 

 conjugated unicellular organisms in that (1) they remain 

 united and work together for the common good, (2) they 

 undergo differentiation of form and specialization of function 

 along certain definite lines of descent (into bone, muscle, gland 

 and other cells), the germ-cell being specialized, for the repro- 

 duction of the complex multicellular organism, and (3) they 

 multiply in different lines of descent at unequal, though definite, 

 rates. Did the cell-descendants of the fertilized ovum all 

 multiply at an equal rate, a solid spherical mass of cells would 

 of course result ; whereas, owing to differences in their rates of 

 multiplication, the shapes of multicellular plants and animals 

 are irregular (i.e. not spherical). But, though these rates of 

 multiplication are pretty definite in every species of plant and 

 animal, they differ widely in different species whence arise 

 differences in shape betwixt one species and another. An 

 ox, for instance, differs in shape from a man because in it the 



1 The nucleus was formerly regarded as a non-essential portion of the 

 germ-cell. It is now beyond doubt that it or rather the chromatin 

 contained within it is the sole " bearer of heredity." Except that they 

 are derived from male and female bodies there is nothing male or female in 

 the nuclei of germ-cells. All this was decisively proved by one of 

 Boveri's experiments. He separated by shaking the nucleus from the 

 ovum of one species of Echinus and then fertilized the enucleated ovum 

 with spermatozoa from another species. The larva which resulted 

 developed the true characters of the last species only. 



