THE DEVELOPMENT OF ORGANISMS 



727 



The n chromosomes in an immature germ-cell occur in pairs, one 



member of each pair being of paternal origin and the other of 



maternal origin. The two members of each chromosome pair come 



into intimate physical contact, one often lying in a line with the 



other, like two rodlets end to end. Then one of the two swings round 



so that they lie one above the other, so closely apposed that the 



n 

 total number appears -. The physical contact of the two homologous 



chromosomes is called sjniapsis, and it may involve interactions of 



moment. Thus, as we shall afterwards notice, a segment of the one 



chromosome may exchange places with a segment of the other 



n 

 ("crossing over"). In each of the - double chromosomes there is a 



median separation of the one from the other, so that rings or double 

 V's are formed, and these arrange themselves at right angles to 



Fig. 115. 



The Crossing Over Process in Two Chromosomes of a "Synaptic Pair". Part 

 of the chromosome coloured white interchanges with part of the chromo- 

 some coloured black. 



the equatorial plane of division. If we suppose n to be 8, there are 

 four half-rings, with their broken ends touching, or four V's with 

 the apices of each limb touching. \Vhat happens then is that division 



occurs, taking off four half-rings or four V's, and leaving the - 



number. Thus if the normal number of chromosomes in an unripe 

 Qg^ be 8, the first polar body takes off 4 and leaves 4. 



If the homologous chromosomes, paternal and maternal in 

 origin, always lay in the same relation to one another in forming 

 rings or apposed V's — if, for instance, the paternal chromosomes 

 were always towards the periphery of the ovum — then the formation 

 of the first polar body would remove all the paternal chromosomes. 

 But it appears to be quite fortuitous whether the first polaody 

 gets two paternal and two maternal chromosomes, or three and one, 

 or one and three — on the supposition that 8 is the normal number. 

 Now there is good reason to believe that each of the chromosomes 

 of a homologous pair carries the same complement of similar genes 



