284 LECTURES ON BIOLOGY 



with protoplasm? What prevents them from producing an embryo 

 independently ? When we were dealing with cell-reproduction 

 we saw that the centrosome underwent important transforma- 

 tions, that it appeared to control all processes, and that most 

 investigators saw in the centrosome the real organ of cell-division. 

 In the maturing of the egg-cells this small but important 

 form becomes apparently degenerate and, in this circumstance, 

 according to Boveri, whose theory is gradually gaining universal 

 recognition, is to be sought the cause of the inability of the ova to 

 divide. For in the act of impregnation the spermatozoon brings 

 into the egg-plasm a new organ of division, a new centrosome, 

 and with that the impediment which hitherto stood in the way of 

 division is removed and development can begin. It must, how- 

 ever, be remembered that this is only an hypothesis with which it 

 is very difficult to reconcile many observations ; still it is the best 

 attempt so far made at explaining this phenomenon. 



In some of the lower crustaceans, in which copulation leads 

 with a high degree of certainty to fertilization, and spermatozoa 

 need only be produced in small numbers, these have retained 

 their typical cell-form. The extraordinary, even unique import- 

 ance of the germ-cells to the preservation of the organic world 

 makes us understand that Nature has made the most compre- 

 hensive preparations to secure their origination and growth. 



The most primitive method of germ-cells is found in the 

 sponges, in which ova and spermatozoa, being distributed through- 

 out the entire body, may originate in the mesodermic tissue. In 

 this case we speak of a diffuse formation of germ-cells. But even 

 in these primitive forms we find special preparations made for 

 the better nutrition of the sex-cells, for each ovum is during its 

 period of growth closely surrounded by a wreath of other cells 

 whose one purpose appears to be to feed their protege and guard 

 it against external influences. In similar manner the young 

 semen-forming cells are cared for in all animals. 



In most of the higher classes of animals, and from the vermes 

 upwards, in all classes, the formation of germ-cells goes on in 

 specific sexual glands, the testes and ovaries. We cannot consider 

 here all the numerous preparations that have been made for the 

 care of the germ-cells, but the final results are in all of them, the 

 same : the formation of healthy, vigorous germ-cells. 



