IMPREGNATION OF THE OVUM. 



It will be convenient again to take Asterias glacialis as the 

 type. The part of the germinal vesicle which remains in the 

 egg, after the formation of the second polar cell, becomes con- 

 verted into a number of small vesicles (fig. 29), which aggregate 



B. 



FIG. 31. SMALL PORTIONS OF THE OVUM OF ASTERIAS GLACIALIS. THE SPERMA- 

 TOZOA ARE SHEWN ENVELOPED IN THE MUCILAGINOUS COAT. IN A. A PROMI- 

 NENCE IS RISING FROM THE SURFACE OF THE EGG TOWARDS THE NEAREST 



SPERMATOZOON; AND IN B. THE SPERMATOZOON AND PROMINENCE HAVE MET. 

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themselves into a single clear nucleus, which 

 toward the centre of the egg and around 

 which, as a centre, the protoplasm becomes 

 radiately striated (fig. 30). This nucleus is 

 known as the female pronucleus. By 

 the action of reagents a nucleolus may be 

 shewn in it. In Asterias glacialis the most 

 favourable period for fecundation is about an 

 hour after the formation of the female pro- 

 nucleus. If at this time the spermatozoa are 

 allowed to come in contact with the egg, 

 their heads soon become enveloped in the 

 investing mucilaginous coat. A prominence, 

 pointing towards the nearest spermatozoon, 

 now arises from the superficial layer of pro- 

 toplasm of the egg, and grows till it comes 

 in contact with the spermatozoon (fig. 31, A 

 and B). Under normal circumstances the 

 spermatozoon which meets the prominence is 

 the only one concerned in the fertilization, 



gradually travels 



1''IG. 32. 1'OKTION 

 OF THE OVUM OF AS- 

 TERIAS GLACIALIS AF- 

 TER THE ENTRANCE 

 OF A SPERMATOZOON 

 INTO THE OVUM, IT 

 SHEWS THE PROMI- 

 NENCE OF THE OVUM 

 THROUGH WHICH THE 

 SPERMATOZOON HAS 

 ENTERED. A VITEL- 

 LINEMEMBRANEW1TH 

 A CRATER-LIKE OPEN- 

 ING HAS BECOME DIS- 

 TINCTLY FORMED. 



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