162 General Biology 



The first group or sheet of cells becomes a hollow sphere called a 

 blastula. The blastula continues growing, which means that this single- 

 layered sphere indents, and this indentation extends into the sphere until 

 two layers of cells are formed. This is called the gastrula stage. Ani- 

 mals having two layers stop growth when this stage is reached, while 

 all higher forms produce a third layer of cells between these two. 



Every living animal passes through one or more of these develop- 

 mental processes. This fact led so many of the early biologists to sup- 

 pose that each developmental stage meant that each one of the higher 

 forms of animals must have sprung from those which stopped in the 

 one- and two-layer stage just beneath the higher form. It means, how- 

 ever, that all living forms pass through a similar state of growth. 1 



Very early in this development of an Qgg, after it begins to grow 

 (fertilization apparently furnishes this growth impulse), certain cells 

 divide much more rapidly than do others. The rapid-growing cells, con- 

 sequently, soon surround the less-rapidly growing ones, thus forming a 

 sort of protecting case or capsule for them. Now, some of the very first 

 cells that are thus protected and grow into the very innermost portions 

 of the growing embryo, are the egg-cells and the sperm-mother cells. 

 This occurs long before one can even distinguish what kind of an animal 

 the embryo is to become. 



It was Professor August Weismann of the University of Freiburg 

 in Baden, who in 1892 gave the world his book, "The Germ-plasm, a 

 Theory of Heredity," which has made us interpret the various facts so 

 far mentioned in a different way from what had been done before. Up 

 to that time men said that the reason a boy so closely resembled his 

 father was because he was "a chip from the old block." Professor 

 Weismann has shown us that this is incorrect, and that both father 

 and son are pieces from the same block. That is, the sex-cells in both 

 mother and father, being a part of the earliest differentiation in the 

 growing embryo as already shown, are really placed in position in the 

 child before he is born, so that a parent, simply considered as a parent, 

 has absolutely nothing whatever to do with the matter, such parent's 

 body acting only as a case or capsule which carries the germ-cell to the 

 next generation. 



This is made clearer when it is remembered that every Ggg in the 

 female is already present at the time of such individual's birth. All that 

 happens during her life is a ripening, or maturing, of such tgg, and 

 fertilization by the male sperm. The sperm-mother-cells which are to 

 divide and form sperm, are already present in the male child when he is 

 born, though they begin to divide only after puberty. 



The sex-cells are, therefore, present at birth in each person, and it 



a It does not follow that because a man builds a school, a barn, and a church, that the church 

 must therefore have first been a school and a barn, even though such builder used exactly the 

 same tools and similar material in the building of each structure; in fact, it would not follow, 

 even though he build the foundation and the first story of each structure exactly alike in each case. 



