46 



THE FROG 



duct accumulate around each one. During sexual union the eggs are 

 shed from the anus of the female, and sperm from the anus of the 

 male mingle with them. If spermatozoa enter the eggs before the 

 jelly envelopes have swollen by absorption of water activation occurs. 

 The second polar body is then formed and amphimixis follows. Since 



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Fig. 30. — Stages in spermiogenesis in the guinea-pig. A, spermatid with early 

 growth of flagellum from one centriole. B and C, showing the centrioles near 

 the nucleus, the accumulation of cytoplasm along the flagellum, and the 

 formation of the acrosome, which is the most anterior part of the spermato- 

 zoon. D, mature sperm, showing head, mid-piece, and tail; the cytoplasmic 



mass has been detached and lost. 



(From F. Meves, 1899, Archiv fiir mikroscopische Aiiatomie, vol. 54.) 



these changes can be seen only in sectioned eggs diagrams should be 

 consulted. 



(f) Examine several eggs which have the jelly well swollen. How 

 is the jelly arranged? These and the subsequent stages should be 

 studied in a watch glass containing enough water to cover the eggs. 

 Use the handlens mainly, and in special cases the lowest magnification 

 of the compound microscope with the mirror turned aside so that 

 illumination is secured by the reflection of light from the surface of 

 the egg. When it is desirable to remove the jelly from a preserved 

 egg, it can be done by rolling the egg on a piece of filter paper. In 

 the living egg the light-colored vegetal hemisphere is the heavier, and 

 the darker surface is always uppermost since the egg turns readily 

 within its enveloi)es. The center of the darker or pigmented animal 

 hemisphere is called the animal pole; the opposite point on the sphere 



