142 THE CHROMOSOMES 
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 
