14 ATLAS OF THE FERTILIZATION AND KARYOKINESIS OF THE OVUM. 



colour as the surrounding protoplasm. The tail of the spermatozoon has been left outside of the egg, and is not 

 seen. The entrance-cone is also not seen since it lies slightly out of the plane of section. 



Plate II. Phototype 6. 



Inward Progress of the Sperm-head. The Entrance-funnel. 



The egg-nucleus is not shown since it lies out of the plane of section. The sperm-head has advanced somewhat 

 into the egg, leaving behind it a deeply stained funnel-shaped area extending to the surface. In life the entrance- 

 cone (not visible in the section) is seen at the base of this funnel ; at its apex is the middle-piece, and from its 

 substance the first rays of the sperm-aster afterwards develop. 



Plate II. PHOTOTYrE 7. 

 Unripe Egg abnormally Fertilized. Polyspermy. Entrance-cones. 



The spermatozoa will readily enter unripe eggs which still contain the large germinal vesicle, before the 

 formation of the polar bodies. Such eggs are incapable of forming a vitelline membrane; they are therefore almost 

 invariably polyspermic, as in the present specimen. The entrance-cones are extremely large and persistent, and the 

 section here reproduced clearly shows three of them, while a fourth lies out of focus on the upper side. 



In such eggs the spermatozoon never penetrates deeply, never completes its rotation, and never develops an 

 aster.^ These peculiarities are of great interest as showing how profoundly the egg-constitution is affected by the 

 changes taking place in the nucleus at the time the polar bodies are formed. 



Plate II. Phototype 8. 



The Entrance-funnel. Sperm-head Rotated 90°. 



The sperm-head has now advanced still further into the egg, and has rotated through about 90°, its longer 

 axis lying nearly parallel to the surface. From its base the entrance-funnel, now well-developed, extends outwards 

 towards the surface. No astral rays can be seen in this section, but they are clearly visible in the next stage, 



Plate III. Phototype 9. 



Origin and Structure of the Sperm-aster. 



A stage immediately following the last in which the astral rays are for the first time visible (cf. Text-fig. IX. 

 D). The sperm-nucleus has rotated rather more than 90°, and is already slightly enlarged, as may be seen by 

 comparison with the sperm-head lying outside the egg at the lower left-hand side. The entrance-funnel is faintly 



1 Mathews has, however, observed that a small sperm-aster may be formed in the case of the starfish. 



