Fig. 13. — Diagram of maturation and fertilisation and first stages of 

 cleavage. (From Prof. H. E. Ziegler, with his kind permission.) The 

 colours have been added. 



1. —The immature ovum, with four double chromosomes, longitudinally cleft; c, centro- 

 some ; ch, chromosomes ; ul, nucleolus. 



2.— First maturation division ; the nuclear spindle has at its equator four groups of 

 tetrads, three of which are visible. 



3 and 4. — Formation of the first polar body (P.B.). In fig. 4 a spermatozoon (sp) is 

 entering. The paternal chromatin is shown throughout in red, the maternal in blue, thi 

 centrosome which is brought in by the spermatozoon is shown in yellow. The ovum-centro- 

 some disappears. 



5.— The formation of the second polar body and the division of the first (1 P.B.). The 

 head of the spermatozoon has formed the male pronucleus {sp). The centrosome introduced 

 by the spermatozoon is surrounded by a clear area and rays. 



6.— The second polar body (2 P.B.) has been set adrift. The first has divided into two. 

 The three polar bodies and the now mature ovum have in their nuclei half the normal number 

 of chromosomes. Thus four are seen in the female pronucleus (f.pn). The centrosome has 

 divided into two. 



7. — The male and female pronuclei (sp and f.pn) have become like one another, and are 

 near together. The centrosomes (c) have become the centres of two large systems of rays. 



8. — The two pronuclei are in contact and are coalescing. 



9. — The nuclei have lost their membrane, and the first segmentation-spindle or cleavage- 

 spindle has been formed, a centrosome lying at each pole. The spindle has the normal number 

 of chromosomes, but each has divided, so that eight pairs are present. 



10. — The egg-cell is dividing. The chromosomes are separated into two groups, each 

 group with eight chromosomes. The centrosome at each pole has divided into two. 



11. — The division or cleavage is complete. The rays have disappeared. The chromosomes 

 are represented by minute vesicles or karyomeres. 



12. — The new nuclei have been constituted by union of the vesicles. The centrosomes lie 

 closely apposed, but will occupy the poles of the spindle at the next division. 



Facing p. 51. 



