190 THE PHYSIOLOGY OF REPRODUCTION 



possess an organisation more complex than many of the simpler 

 Metazoa. Moreover, many animal eggs, such as those of the 

 Cephalopod Loligo, and many Arthropod eggs, begin to assume the 

 shape of the future larval form through which they will subsequently 

 pass in their ontogenetic history before any cleavage has taken place, 

 clearly showing preformation in this respect. 



In many eggs, again, the entrance of the sperm is followed 

 by rapid changes in the viscosity of the cytoplasm, followed by 

 streaming effects within the egg. In the Ascidian Cynthia, Conklin a 

 has shown that the cytoplasm of the ripe egg consists of a grey yolky 

 substance occupying the centre of the egg, and surrounded by a 

 peripheral layer of bright yellow pigmented cytoplasm, while at the 

 animal pole is an area of clear cytoplasm, the germinal vesicle. On 

 the entrance of the spermatozoon, an immediate rearrangement of 

 these materials takes place. The yellow cytoplasm now streams 

 down to the negative pole of the egg and arranges itself symmetri- 

 cally with regard to the egg axis, while the clear cytoplasm of the 

 germinal vesicle comes to lie above it. The grey yolk now lies 

 at the animal pole containing the female pronucleus embedded 

 in it. 



That the entrance of the sperm into the egg of the sea-urchin 

 initiates some structural mechanism in the egg, by virtue of which 

 its high rate of oxidation in the fertilised condition is rendered 

 possible, would seem to be borne out by experiments of Warburg 

 and Meyerhof. 2 They found that, if they ground up the eggs 

 with sand and sea- water to a tine paste, the unfertilised eggs 

 consumed just as much oxygen in the broken condition as when 

 intact and whole. There was only a slight and very insignificant 

 fall in the respiratory quotient. If the fertilised eggs were reduced 

 to a paste, then, instead of this having the normal respiratory 

 quotient characteristic of the fertilised egg, it was invariably a third 

 or fourth of this amount. Thus the rubbing up with sand made 

 little difference in the case of the unfertilised egg, but was followed 

 in the fertilised egg by an enormous drop in the respiratory quotient. 

 That this mechanism is not bound up in any way with the usual 

 visible structure of the egg is shown by further experiments, in 

 which the eggs were treated with acetone. It was found that 

 acetone is an excellent fixative for the egg, preserving the minute 

 details of microscopical structure and chemical composition 

 unchanged ; even the egg lipoids seemed unaffected by the use of 

 acetone. In the case of the unfertilised egg, not a great deal of 



1 Conklin, "The Organisation and Cell Lineage of the Ascidian Egg," 



\'-"d. Nat. Science P/n'td., vol. xiii., 1905. 



'-' Warburg and Meyerhof, "Tiber Atmung in abgetoteten Zellen und 

 Xfllenfraginenten,' .l/v/,. /. ,/,-.<. /'/^W., vol. cxlviii., 1912. 



