MATT RATION OF THE EGG. 11 



polar bodies or not. The point is of considerable interest, for on 

 the one view the formation of a polar body would be merely the 

 division of the egg cell into two very unequal portions, accom- 

 panied by the ordinary phenomena of mitosis ; while on the 

 other view the process would be of a very unusual character, 

 consisting in nuclear division, with extrusion of one of the 

 daughter nuclei. The actual details of the process, and more 

 especially a comparison with corresponding changes that occur 

 in plants, strongly support the former view, that the formation 

 of polar bodies is an act not merely of nuclear division, but of 

 true cell division. 



6. The polar bodies play no part whatever in the develop- 

 ment of the embryo. They persist for some time after their 

 formation, but ultimately disappear completely. 



7. In the great majority of cases that have been studied, 

 including representatives of almost all the great groups of 

 Metazoa, two polar bodies are formed in succession, as described 

 above. The first polar body, after its separation from the egg, 

 not uncommonly divides into two, giving three polar bodies in 

 all. Weismann and Bloclimann have shown that the eggs of 

 certain Entomostraca, and of Aphis, which develop partheno- 

 genetically, i.e. without requiring fertilisation by the male 

 element or spermatozoon, form only one polar body. 



8. After extrusion of two polar bodies an egg appears to be, 

 as a rule, incapable of developing into an embryo unless and 

 until it is fertilised by a spermatozoon. The rule is not absolute, 

 for at least two exceptions are known : the eggs of the gipsy moth, 

 Liparis dispar, and the eggs of the hive bee from which drones 

 are developed, are stated to extrude two polar bodies, and yet 

 to develop without being fertilised. 



The above facts indicate that there is a close, though not in 

 all cases a necessary connection between the formation of polar 

 bodies and the act of fertilisation ; and the further consideration 

 of the matter may well be postponed until the latter process has 

 been described. 



Fertilisation of the Egg. 



With certain exceptions which will be noted further on, an 

 egg before it can commence to develop into an embryo requires 

 to be fertilised. 



