THE DEVELOPMENT OF ORGANISMS 731 



divisions of the egg-cell, and to show that each cell has half of its 

 chromosomes of paternal origin and the other half maternal; and to 

 this must be added the statement that the chromosomes form 

 homologous pairs. That is to say, if there be a chromosome (A) of 

 maternal origin that carries the genes or factors for a certain group 

 of characters, there will be another chromosome (A') of paternal 

 origin that carries the genes or factors for the same group. It follows 

 that Huxley was right in his extraordinary prevision of 1878: "It 

 is conceivable, and indeed probable, that every part of the adult 

 contains molecules derived both from the male and from the female 

 parent; and that, regarded as a mass of molecules, the entire 

 organism may be compared to a web of which the warp is derived 

 from the male and the woof from the female." "What has since 

 been gained". Prof. E. B. Wilson says, "is the knowledge that this 

 web is to be sought in the chromatic substance of the nuclei, and 

 that the centrosome is the weaver at the loom." 



To sum up, let us answer the question: What is implied in the 

 fertilisation of the ovum by the spermatozoon? (i) There is a mingling 

 of two inheritances, paternal and maternal, slightly different from 

 one another, the one mainly carried by the chromosomes of the 

 spermatozoon and the other mainly carried by the chromosomes of 

 the egg-cell. To many biologists, it remains an open question 

 whether the extra-nuclear protoplasm carries any hereditar^^ factors; 

 but it must be noticed here that the egg-cell furnishes in its cyto- 

 plasm a relatively large quantity of embryonic building material, 

 whereas the spermatozoon has almost none. 



(2) The number of chromosomes, reduced to a half by meiosis, 

 is restored to the normal in fertilisation. In some cases the normal 

 number is different in the two sexes ; thus it is 47 in man and 48 in 

 woman — an interesting difference referred to in the section on sex- 

 determination. We shall simply re-emphasise the fact that the 

 number itself is not so important as the adherence to it throughout 

 all the cells of the body. 



(3) There is an introduction of a centrosome by the middle piece 

 of the spermatozoon, or else there is some stimulus which evokes the 

 appearance of a new centrosome in the cytoplasm of the egg-cell. 



(4) There is an excitation of the cytoplasm of the ovum, which 

 seems to pass from the pole where the sperm enters. This may 

 imply an exudation between the vitelline membrane and the surface 

 of the ovum, so that the membrane is lifted off to form the fertilisa- 

 tion membrane. There is in most cases a "blocking" of the egg to 

 other sperms. 



(5) But fertilisation also involves a re-activation of the egg, a 

 rearrangement of materials, a stimulus to divide; and the first 

 cleavage plane is determined by the entrance of the spermatozoon, 

 and by the path taken through the cytoplasm by its nucleus 



