EMBRYOLOGY 



211 



Furthermore, the pairing is not a purely fortuitous occurrence, for each 

 paternal chromosome meets with a particular maternal chromosome. As 

 a result of this union of the chromosomes there are of course half as many 

 pairs as there- were chromosomes before. 



Tetrads.^ — The pairs of chromosomes often do not appear as doul^le 

 bodies; for while the chromosomes have been coming together they may 



Sper/Jiafozoo. 



lit Polar Body. 



(P^^^SecondarLj 

 Oocyte. 



Dyads. 



2ad Polar Bodij- 

 Mature Egg. 



pertilized Egg. 



First Ckayoge. 



Fig. 164. — Maturation of the germ cells, diagrammatically represented for both 

 sexes. The black chromosomes may be assumed to be of paternal origin, the white ones 

 maternal. 



also have divided. .Each pair thus consists of four half -chromosomes; 

 and the quadruple body formed is called a tetrad. Owing to its oi-igin, two 

 of the parts of each tetrad are maternal, the other two paternal. In the 

 two maturation divisions the tetrads are divided, in two planes, first into 

 double bodies called dyads, next into their single components. 



First Maturation Division.— A spindle *is formed, on which the tetrads 

 take their place. How the tetrads are divided depends on the way they 

 are placed on the spindle. In part, this position appears to be fixed and 



