220 ZOOLOGY. 



i 



nant character and the other half the recessive. It must 

 not be supposed that in the reduction divisions the whole 

 of the chromosomes received from the male parent are 

 separated from those received from the female, for if this 

 were the case the different individuals in the second gener- 

 ation of a cross would resemble one or other of the original 

 crossed grandparents in all characters, and this does not 

 happen. What has been said above of a Mendelian cross 

 only applies to one pair of characters ; when two or more 

 pairs are considered the matter becomes much more com- 

 plicated. It follows from the Mendelian theory that we 

 must suppose segregation to take place in ordinary repro- 

 duction, but in this case the same characters occur in both 

 parents and in both of a pair of segregated gametes. 

 Thus we may represent a Mendelian experiment as 

 follows : 



Grossed parents DT> and EE. 



Gametes in these D -{- D, E + E, which conjugate. 



Zygotes D E, the individuals of first generation. 



Gametes of these D -f E and D -f- E, which again 

 conjugate. 



Zygotes DD + 2DE -f !^E, individuals of second 

 generation. 



The individuals developed from unlike gametes are 

 called hetero-zygotes, those from like gametes homo- 

 zygotes. But an individual is not necessarily heterozy- 

 gous or homozygous for all characters. Each pair of 

 characters behaves as a rule independently. 



In parthenogenesis when the egg develops without fertili- 

 sation there is probably no reduction of chromosomes, and 

 therefore no segregation : but this is not certain. 



6. Segmentation. Usually after a very short interval 

 the fertilized ovum begins to divide, and the resulting cells 

 divide again and again. In these and in all cases of cell- 

 division the nucleus divides first. At first the cells produced 

 are all much alike, and the term segmentation is applied 

 to this continued cell-division during the earliest stages, 

 before the differentiation of cells begins to become so im- 

 portant that the mere cell-divisions are not noted. But 



