536 MATURATION AND IMPREGNATION OF THE OVUM. 



and by Butschli, we have instances of a different mode of for- 

 mation of the polar cells. 



The case of Amphibians, as described by Bambeke (2) and 

 Hertwig (12) cannot so far be brought into conformity with our 

 type, though observations are so difficult to make with such 

 opaque eggs that not much reliance can be placed upon the exist- 

 ing statements. In both of these types of possible exceptions it 

 is fairly clear that, whatever may be the case with reference to 

 the formation of the polar cells, part of the germinal vesicle 

 remains behind as the female pronucleus. 



There are a large number of types, including the whole of the 

 Rotifera J and Arthropoda, with a few doubtful exceptions, in 

 which the polar cells cannot as yet be said to have been satis- 

 factorily observed. 



Whatever may be the eventual result of more extended inves- 

 tigation, it is clear that the formation of polar cells according to 

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

 very greatly increased by the discovery by Strasburger of the 

 existence of an analogous process amongst plants. Two questions 

 about it obviously present themselves for solution : (i) What are 

 the conditions 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 

 impregnation, 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 shew that in the later 

 cases the process is influenced by the contact with the male 

 element. In Asterias, as has been shewn by O. Hertwig, the 



1 Flemming (6) finds that, in the summer and probably parthenogenetic eggs of 

 Lacinularia socialis, the germinal vesicle approaches the surface 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 Flemming believes a polar cell to 

 be situated, though he has not definitely seen one, 



