THE CHROMOSOMES 109 



egg, in other cases it appears to enter and to be 

 absorbed. 



Behind the head of the spermatozoon, and at the 

 base of the tail, there is a middle piece which contains 

 a derivative of the old centriole or division center. 

 Since the centrosome carried by the sperm has been 

 found in some forms to give rise to the new centro- 

 somes that occupy the poles of the first cleavage spindle 

 of the egg, it may appear that a paternal contribu- 

 tion can come about in this way. It is true that the 

 continuity of the centrosome of the sperm with that 

 of the dividing egg has been disputed in some forms; 

 but it is difficult to prove that the sperm centrosome 

 is lost, even though it may disappear owing to loss 

 of staining power. 



The nucleus contains a sap which is probably of 

 cytoplasmic origin. The presence of this sap may 

 again be appealed to by those who do not accept 

 the chromosomes as the bearers of heredity, as a 

 weak link in the evidence. It is true that the nuclear 

 sap appears to be squeezed out of the nucleus of the 

 sperm head, leaving a compact and apparently solid 

 mass of chromatin, yet its complete elimination can 

 not be proved. Hence, while those who favor 

 chromosomal transmission find in the facts of normal 

 fertilization strong indications favorable to that 

 view, yet it is also true that those who are inclined to 

 dispute this view find several loopholes in the 

 argument of their opponents. 



The importance of the nucleus in heredity has 

 further been shown by experiments of Bierens de 



