CHAPTER II. 



THE MATURATION AND IMPREGNATION OF THE OVUM. 



Maturation of the ovum and formation of the polar bodies. 



IN the preceding chapter the changes in the ovum were described 

 nearly up to the period when it became ripe, and ready to be 

 impregnated. Preparatory to the act of impregnation there take 

 place however a series of remarkable changes, which more especially 

 concern the germinal vesicle. 



The attention of a large number of investigators has recently been 

 directed to these changes as well as to the phenomena of impregnation. The 

 results of their investigations will be described in the present chapter; but 

 for an historical account of these in- 

 vestigations, as well as for a deter- 

 mination of the delicate questions 

 of priority, the reader is referred to 

 Fol's memoir (No. 87), and to a 

 paper by the author (No. Si). 



The nature of the changes 

 which take place in the matura- 

 tion of the ovum may perhaps 

 be most conveniently displayed 

 by following the history of a sin- 

 gle ovum. For this purpose the 

 eggs of Asterias glacialis, which 

 have recently formed the subject 

 of a series of beautiful researches 

 by Fol (87), may be selected. 



The ripe ovum (fig. 22), when 

 detached from the ovary, is form- 

 ed of a granular vitellus enve- 

 loped in a mucilaginous coat, the 

 zona radiata. It contains an eccentrically-situated germinal vesicle and 



FIG. 22. RIPE OVUM OF ASTERIAS GLACI- 

 ALIS ENVELOPED IN A MUCILAGINOUS ENVELOPE, 

 AND CONTAINING AN ECCENTRIC GERMINAL VE- 

 SICLE AND GERMINAL SPOT ( Copied from Fol). 



