Chromatin as *' Hereditary Suhstance^^ 343 



ted tlicat in no case among animals, so far as known, is the 

 sperm head derived quite exclusively from the chromosomes. 

 A small amount of the cytoplasmic part of the spermatid 

 appears always to be carried on into the spermatozoan as 

 a surface layer of the head. And the "middle-piece" or 

 part immediately beliind the head, seems always to contain 

 material not derived from the chromosomes. We shall have 

 to examine these extra-chromatinic portions of the sperm 

 more fully when we undertake to find what substances, 

 whether in germ or somatic cells, participate directly in 

 actual development. 



In the meantime we must recognize the important part 

 taken by the chromosomes, or more exactly by chromatin, in 

 fertilization and in the first steps of development of the 

 individual. The evidence is especially weighty in some of 

 the higher plants where according to one eminent botanist, 

 Eduard Strasburger, the nucleus only of the pollen- grain 

 enters the ovum. Summing up the results on the point, 

 Strasburger writes, "In these plants (the flowering plants) 

 the male sexual cells lose their cell-body in the pollen-tube 

 and the nucleus only — the sperm nucleus — reaches the egg. 

 The cytoplasm of the male sexual cell, is therefore not neces- 

 sary to ensure a transference of hereditary characters from 

 parent to offspring. I lay stress on the case of the Angio- 

 sperms because researches recently repeated with the help 

 of the latest methods failed to obtain different results." ^^ 

 Should this statement receive confirmation by future investi- 

 gation it would mark the flowering plants as the group of 

 organisms in which specialization has gone farther than in 

 any other so far known toward making chromatin the sole 

 genetic intermediary between male parent and offspring. 



But the sperm head, composed almost exclusively of chro- 

 matin, unites with chromatin only of the female germ-cell, 

 the quantity of the male chromatin being apparently equal 

 to that of the female chromatin. These facts are clearly very 



