56 LECTURES ON BIOLOGY 



Forthwith the more favoured germ-cells take advantage of the 

 unfortunate position of their brothers ; numerous normal and 

 healthy spermatozoa penetrate with their heads into the disin- 

 tegrated cells and commence to feed vigorously and to grow. At 

 no stage of their development are the germ- cells quite secure 

 against destruction, for it may not infrequently be observed that 

 almost fully developed spermatozoa disintegrate and are assimilated 

 by the survivors. It is a veritable struggle for existence in which 

 apparently the fittest gain the victory here as elsewhere in Nature. 



This secret fight to the death is of double importance for the 

 species. In the first place, it is only the healthy, vigorous sper- 

 matozoa which will reach the final stage of development, and 

 these, again, are during the entire period of growth abundantly 

 supplied with food. But vigorous and healthy germ-cells are 

 the surest guarantee of a vigorous generation, and a rational 

 result is thus obtained by a crude, mechanical selection of the 

 strongest cells. 



But let us return to the starting point of this examination, 

 the conditions observed in the femur. Here, too, we see that 

 the purposeful arrangement of the individual pillars and arches 

 proceeds from a struggle between the individual bone-forming 

 cells. We have already seen that an increased stimulus conse- 

 quent upon an increased normal function of an organ or part of 

 an organ leads to an increase in assimilation and a strengthen- 

 ing and better formation of the affected tissues. The bone, 

 however, does not consist of a homogeneous substance but of 

 heterogeneous elements ; in this case, therefore, the burden 

 placed upon the femur is not equally distributed but stimulates 

 different parts to a different degree. It is therefore quite natural 

 i,hat the largest amount of bone-tissue is formed and deposited 

 in the direction of the strongest pressure, for there the bone- 

 forming cells are most frequently and intensely stimulated into 

 activity and growth. The interspersed parts, however, which 

 participate in the work but slightly or not at all, suffer a corre- 

 sponding want of nutrition and lag behind in their development. 

 In this way we obtain a typical pillar-and-arch structure, as 

 shown in the picture, reminding us of the rational plan of the 

 expert engineer. 



In one respect, however, the natural structure far sur- 



