330 BIOLOGY 



animals that multiply by simple division, or by budding, ex- 

 plains itself. 



Reproduction by Eggs or by Spores. When we come to 

 the reproduction of the multicellular animals and plants by 

 eggs and spores, the problem, however, becomes more difficult. 

 The egg or the spore is a single cell, and from this single cell 

 develops the many-celled adult. When this cell divides into 

 many cells, which become differentiated and form themselves 

 into new individuals, why should these adults be repetitions 

 of the parent? There can be only one answer. This single cell, 

 whether an egg or a spore, must contain in itself, in some form 

 or other, features representing the whole of the animal from 

 which it came. We may place two eggs in an artificial incu- 

 bator and hatch them by artificial heat under identical condi- 

 tions; one of them becomes a duck and the other a chick. It 

 is absolutely impossible to avoid the conclusion that in one of 

 the original eggs there were present potentially the characters 

 which would produce the duck, and in the other the charac- 

 ters which would produce the chick. This of course indicates 

 a complexity in the egg far beyond the possibility of our imag- 

 ination. But we are logically forced to the conclusion that the 

 facts are as stated. An egg or a spore undoubtedly contains 

 potentially all of the characters of the animal into which it 

 develops. 



Germ Plasm. For convenience in discussion it is agreed to 

 call this substance, which is present in the egg and contains 

 the hereditary characters, by the name of germ plasm. We 

 have seen, in Chapter XII, reasons for believing that this mate- 

 rial is chiefly, if not wholly, confined to the part of the egg that 

 we have called the chromatin. We have also learned that the 

 chromatin is capable of growing and dividing and has the power 

 of self-perpetuation. Using the term "germ plasm" for this 

 material that possesses the power of determining the develop- 

 ment of a new individual, it follows that the germ plasm has 

 the power of growth. In other words, this germ substance, 



