THE FROG, A TYPICAL VERTEBRATE 



263 



digit of their fore legs. The sex organs within the bodies of the 

 two sexes are very different; they are essentially like those in all 

 other vertebrates including man. 



The germ cells of the male, the spermatozoa, arise in two oval 

 organs, the testes (Fig. 149, A, 13). When mature, they pass 



FIG. 150. Diagrams representing the essential phenomena of mitosis. 



A, a cell with resting nucleus ; B, the centrosomes are separating ; tlw 

 chromatin forms a convoluted thread or spireme ; C, the spireme is broken 

 up into a number of V-shaped chromosomes ; D, the chromosomes are ar- 

 ranged at the equator of the spindle; E, division of the chromosomes; F, di- 

 vergence of the chromosomes ; G, chromosomes collecting at the poles of the 

 spindle ; commencement of division of the cell body ; H, I, complete division 

 of the cell, and reconstitution of the nuclei. (From Bourne.) 



through tubules (3) into the kidney (12) and from there through 

 the kidney ducts, the ureters, to the seminal vesicle (5), where 

 they are stored until the eggs of the female are ready to be fer- 

 tilized. Then they pass out through ths anil opening. 



