122 F. M. BALFOUR. 



the polar cells cannot as yet be said to have been satisfactorily 

 observed. 



Whatever may be the eventual result of more extended inves- 

 tisration, it is clear that the formation of polar cells accord into our 

 type is a very constant occurrence. Its importance is also very 

 ggreatly increased by the discovery by Strasburger of the existence 

 of an analogous process amongst plants. Two questions about it 

 obviously present themselves for solution : (1) What are the con- 

 ditions of its occurrence with reference to impregnation ? (2) What 

 meaning has it in the development of the ovum or the embryo? 



The answer to the first of these questions is not difficult to 

 find. The formation of the polar bodies is independent of im- 

 pregnation, and is the final act of the normal growth of the 

 ovum. In a few types the polar cells are formed while the ovum 

 is still in the ovary, as, for instance, in some species of Echini, 

 Hydra, &c., but, according to our present knowledge, far more 

 usually after the ovum has been laid. In some of the instances 

 the budding off of the polar cells precedes, and in others follows 

 impregnation ; but there is no evidence to show that in the later 

 cases the process is influenced by the contact with the male 

 element. In Asterias, as has been shown by O. Hertwig, 

 the formation of the polar cells may indifferently either precede 

 or follow impregnation —a fact which affords a clear demonstration 

 of the independence of the two occurrences. 



To the second of the two questions it does not unfortunately 

 seem possible at present to give an answer which can be regarded 

 as satisfactory. 



The retrogressive changes in the membrane of the germinal 

 vesicle which usher in the formation of the polar bodies may very 

 probably be viewed as a prelude to a renewed activity of the 

 contents of the vesicle ; and are perhaps rendered the more neces- 

 sary from the thickness of the membrane which results from a 

 protracted period of passive growth. This suggestion does not, 

 however help us to explain the formation of polar cells by a pro- 

 cess identical with cell division. The ejection of part of the 

 germinal vesicle in the formation of the polar cells may probably 

 be paralleled by the ejection of part or the whole of the original 

 nucleus which, if we may trust the beautiful researches of liiit- 

 sclili, takes place during conjugation in Infusoria as a preliminary 

 to the formation of a fresh nucleus. This comparison is due to 

 Biitschli, and according to it the formation of the polar bodies 

 would have to be regarded as assisting, in some as yet unknown 



and becomes invisible, and that subsequently a slight indentation in the 

 outline of the egg marks the point of its disappearance. In the hollow 

 of the indentation Kleniming believes a polar cell to be situated, though he 

 has not definitely seen one. 



