130 The Science of Life. 



conclusions is that of Boveri, who maintains that the 

 ripe egg possesses all the organs and qualities necessary 

 for division excepting the centrosomes, by which division 

 is initiated. The spermatozoon, on the other hand, is 

 provided with a centrosome, but lacks the substance in 

 which this organ of division may exert its activity. 

 Through the union of the .two cells in fertilization all of 

 the essential organs necessary for division are brought 

 together; the egg now contains a centrosome which by 

 its own division leads the way in the embryonic develop- 

 ment. This is not the place to attempt a discussion of 

 a very difficult problem, but we may cite the summing 

 up given by one of the clearest of modern exponents 

 Prof. E. B. Wilson. ''From the mother comes in the 

 main the cytoplasm of the embryonic body, which is the 

 principal substratum of growth and differentiation. 

 From both parents comes the hereditary basis or chro- 

 matin by which these processes are controlled, and from 

 which they receive the specific stamp of the race. From 

 the father comes the centrosome to organize the ma- 

 chinery of mitotic division by which the egg splits up 

 into the elements of the tissues, and by which each of 

 these elements receives its quota of the common heri- 

 tage of chromatin. Huxley hit the mark twoscore 

 years ago when he compared the organism to a web of 

 which the warp is derived from the female and woof 

 from the male. What has since been gained is the 

 knowledge that this web is to be sought in the chro- 

 matic substance of the nuclei, and that the centrosome 

 is the weaver at the loom." 



The segmentation of the egg leads on to the estab- 

 lishment of the two primary germinal layers the 

 Germinal ectoderm or epiblast, and the endoderm or 

 Layers. hypoblast. These layers are established in 

 different ways in different types, but on the whole 

 they give rise to similar structures throughout. The 

 ectoderm forms especially the epidermis, the nervous 

 system, and the foundations of the sense-organs, and a 

 region at each end of the food-canal (fore-gut and mid- 

 gut); the endoderm forms especially the lining of the 

 mid-gut, and of the outgrowths which arise from it, and 



