APPENDIX 495 



3. Gradients and prelocalisation in Polychaetes {Nereis). 



The work of Spek (Protoplasma, ix, 1930, p. 370; and Parts iv 

 and V of Gellhorn's Lehrbtich der allgemeinen Physiologic, 1931) on 

 this subject presents such a number of features of interest, that it 

 deserves special mention here. To the technique of intra vitam 

 staining of the early stages of the developing egg of Nereis, he has 

 added the use of indicators enabling him to detect changes of pYl 

 in different parts of the embryo. The unfertilised tgg of Nereis 

 contains a large germinal vesicle (nucleus) surrounded by a number 

 (about 30) of drops of fat arranged around it in the equatorial plane ; 

 in addition, there is, scattered through the tgg, 3. large number of 

 small albuminous droplets containing a lemon-yellow pigment. 

 This pigment is found to turn violet in acid media and therefore 

 acts as a natural indicator. 



Fertilisation results in the formation of the polar bodies at the 

 animal pole, as well as in a number of changes in the cortical 

 regions of the cytoplasm which do not directly concern us here. 

 What is of great interest, however, is the fact that soon after the 

 polar bodies are extruded, the fat drops and albumen droplets 

 undergo a re-arrangement, as a result of which they leave a clear 

 zone or "pole-plasm" (see p. 113) at the animal pole and become 

 concentrated in the vegetative hemisphere of the egg (see p. 119 

 for a comparable case of re-arrangement and prelocalisation of egg- 

 contents in Ascidians). The albumen droplets in the equatorial 

 zone take on a peculiar colour when stained intra vitam, and the 

 whole egg shows a clear stratification along the main axis. 



The egg is in this condition when the first two (meridional) 

 cleavage-divisions take place, leaving four blastomeres, each pos- 

 sessing the characteristic stratification. There next occurs a remark- 

 able change in the pH of different regions along the main axis, as 

 shown by the natural and by the experimentally added indicators. 

 The region of the animal pole shows a shift to the alkaline, while 

 the region of the vegetative pole shows a shift to the acid side of the 

 scale. 



The whole phenomenon, as Spek says, presents the appearance 

 of a natural experiment of cataphoresis. The changes in pH at the 

 two ends of the main axis of the egg reflect a gradient in electrical 



