6 LABORATORY OUTLINES FOR EMBRYOLOGY 



2. Maturation 



a. While the egg is in the Graafian follicle the nucleus 

 is large and the chromatin is in the so-called resting con- 

 dition. The egg is said to be in the growth period. At 

 the end of the growth period it passes through two suc- 

 cessive, modified mitotic divisions, the maturation divi- 

 sions. In the eggs of some animals the maturation 

 divisions occur before the egg leaves the ovary, but in 

 others they do not occur until afterward. The Ascaris 

 egg is of the latter type. 



On the slide of Ascaris find an egg in the growth period. 

 Draw it and label all the parts. 



b. At the end of the growth period, there are half as 

 many chromatin masses as there are somatic chromo- 

 somes. In Ascaris magalocephala bivalens there are four 

 somatic chromosomes and, therefore, two chromatin 

 masses at the end of the growth period. Each chroma- 

 tin mass is made up of four parts and is called a tetrad. 

 The tetrads arrange themselves on a spindle which is 

 comparatively small and is eccentric in position. 



Draw an egg with the tetrads in the middle vf the spindle 

 (metaphase) and label all the parts. 



c. Each tetrad divides into two parts called dyads. 

 One dyad from each tetrad moves to one pole of the 

 spindle and the other dyad moves to the other pole of the 

 spindle. 



When the dyads reach the poles the egg divides. The 

 resultant cells are very unequal in size. One cell con- 

 tains half of the chromatin and little of the cytoplasm. 



