294 DARWINISM AND THE PROBLEMS OF LIFE 



instance. The particle of protoplasm that constitutes 

 the animal contracts from two opposite sides. It 

 becomes thinner and thinner in the middle, until the 

 cord that has kept the two halves together gives way. 

 During the division the nucleus also has been drawn 

 out and constricted, so that when two animals are found 

 instead of the one, each of them has a half of the 

 nucleus. The two pieces of the amoeba then quickly 

 feed themselves up to the normal size of the species, 

 and each half of the nucleus attains the size of the 

 whole one. The essence of reproduction is the same 

 in all the protozoa : the animal splits into two halves, 

 and thus the "mother" divides into two "daughters." 



The cells of the higher animals multiply in the same 

 way as the protozoa. The only difference is that in 

 their case the cells remain united after the cleavage, 

 whereas in the protozoa they become independent, and 

 go their own ways. Hence the development of a multi- 

 cellular animal is as follows. The ovum, which always 

 consists of one cell, divides into two daughter-cells ; 

 these again divide into two, and thus four are produced. 

 After the next cleavage there are eight, then sixteen 

 cells, and so on, until we have a compact cluster of 

 cells. The cells then arrange themselves ; they lay 

 themselves in two strata, and form a sac. Thus they 

 reach the polyp stage, and the differentiation continues. 

 Some of the cells secrete muscular matter, others the 

 skeletal matter, until the whole complicated animal is 

 constructed. 



What a wonderful development ! How is it possible 

 that from this repeated cleavage we get, not an irregular 



