234 



ESSENTIALS OF BIOLOGY 



As the nucleus divides one-half of each tetrad goes into each 

 daughter-nucleus . 



Now it is a matter of chance which way the tetrad is situated 

 in the mother cell before the heterotype division occurs. Thus 

 we may have the divisions 5 and 6 (in Fig. 30). 



Here we have two cases : the *' ^ "s in the first case (2) are 



A a 



-2 but in the second one (5) they are - and so on. Thinking over 



the matter the student will find that there are sixteen ways in 

 which (by random now) the nucleus containing the tetrads can 

 divide so that each daughter-nucleus receives an " out-fit," '' A " 

 *' 5 " '' C " " Z)," but the outfits will differ from each other in 

 that they contain different combinations of maternal and paternal 

 chromosomes. 



The dyads now divide just as in an ordinary mitosis, thus : 



*(?) 



Fig. 31. — Division of a Dyad. 



and each gonidial cell will thus give rise to four daughters. If 

 the gonidial cell is an ovum-mother-cell one of the daughters 

 becomes the ovum and the other three become the (abortive) 

 '* polar bodies." If the gonidial cell is a sperm-mother-cell all 

 four daughters become functional spermatozoa. 



816. The Gametes. The gametes are the cells that con- 

 jugate, the ova and spermatozoa. It will be seen from the above 

 summary account of maturation that there are (if the haploid 

 number of chromosomes is 4) 16 different kinds of ova and as 

 many kinds of spermatozoa. There are, for instance, the ova, 

 A B C D, a B C D, ah C D, etc. All these kinds of ova (and 

 spermatozoa) contain the developmental ensemble E and the 

 " outfit " " ^," " 5 " " C " " or The ensemble E is (by 

 hypothesis) the '' outfit " that is responsible for the development 

 of the specific (and constant) characters and the " outfit " " A " 

 " B " '* C " " i) " is responsible for the development of the 

 *' loose " Mendelian characters. 



