314 LECTURES ON BIOLOGY 



organic development is thereby driven from its isolated position 

 and brought into intimate correlation with the entire genesis of 

 life. The development of the ovum becomes a little wheel in the 

 whole wonderful engine of Nature. 



We heard before that during fertilization only one spermato- 

 zoon invades the egg, the entrance being immediately closed to 

 the others. In immediate connection with the centrosome, 

 which has been carried into the ovum by the spermatozoon, 

 is formed the normal bi-polar ' spindle-figure ' which divides the 

 nucleus with the accuracy of the most perfect apparatus into 

 two daughter-nuclei equal in volume and value. As Boveri 

 showed, this accurately working apparatus of division suffers 

 serious disturbances when accidentally two spermatozoa simul- 

 taneously arrive within the ovum. For each of the two centro- 

 somes behaves as if it were alone in the ovum, and in place of 

 the normal ' spindle ' we observe a figure of four poles. Accord- 

 ingly the egg divides immediately into four cells, but as in such 

 a case the ' nuclear loops ' of three nuclei must be distributed 

 among four daughter-nuclei, none of the nuclei obtains the 

 number due to it. Though the vitality is not thereby destroyed, 

 and though the ova continue to develop, the larvae which emerge 

 exhibit varying defects corresponding to the incomplete store of 

 chromosomes. This phenomenon suggests that the individual 

 nuclear loops are not equivalent to each other, but that to each 

 chromosome is due only part of the ' rudiments ' of a complete 

 organism, and that a well-defined selection of nuclear loops is 

 necessary for a normal development. The facts of partheno- 

 genesis and ephebogenesis, when considered with this observation, 

 permit us, therefore, to conclude that the mature ovum as well 

 as the spermatozoon each contain exactly the material for one 

 complete organism. 



But while generally no distinction can be made between the 

 individual chromosomes of the nucleus their unequal character 

 is in certain cases distinctly shown by their appearance. Thus 

 the nucleus of the male primary germ-cells of a species of locust, 

 Brachystola magna, contains, according to Button's investigations, 

 twenty-four nuclear loops which may be easily distinguished in 

 form and size : eighteen large and six small chromosomes. These 

 may be still further subdivided into twelve degrees of size, so 



