NOVEMBEB 12, 1909] 



SCIENCE 



695 



as is shown both by its insolubility in concen- 

 trated mineral acids, including sulphuric, and 

 also on short boiling in molecular solutions of 

 caustic alkalis. On prolonged boiling it 

 either dissolves or becomes so broken up as to 

 be unrecognizable. The egg itself dissolves 

 entirely in concentrated H^SO, and in NaOH 

 except for a few granules. Unfertilized eggs 

 dissolve entirely in concentrated H,SOj, show- 

 ing that the membrane is not present before 

 fertilization. It is also left undigested by 

 pepsin HCl. 



Regarding the membrane as a secretion, its 

 formation is strong evidence that an increase 

 of permeability, of which it is the direct re- 

 sult, is brought about by the various mem- 

 brane-forming substances. Its removal from 

 the sphere of reacting substances (in the egg) 

 must upset any chemical equilibrium which 

 has been attained, this equilibrium meaning a 

 condition of rest and non-development of the 

 egg. 



The second visible change occurring in some 

 sea-urchin eggs, e. g., Arhacia, is the migration 

 of the red pigment granules, which, until after 

 formation of the fertilization membrane are 

 distributed throughout the cytoplasm, to the 

 periphery of the egg, as mentioned by Mc- 

 Clendon. This migration can be explained 

 on the assumption that the change of permea- 

 bility associated with membrane formation is 

 connected with ionic interchange between ex- 

 terior and interior of the cell giving rise to 

 potential differences such as are seen in the 

 functioning of glands, muscles, nerves and 

 sensitive plants. Lillie' has discussed this, 

 theoretically, in a paper in which an increase 

 in permeability is also taken to be the change 

 bringing about development. " With the 

 appearance of an increased permeability 

 . . ., the peripheral regions of the proto- 

 plasm must become, for a time at least, until 

 the potentials are equalized, positive rela- 

 tive to the interior." Most small particles 

 suspended in a fluid become negatively charged 

 and migrate in an electric field. The fact that 

 these bodies are repelled by the asters is 

 further evidence for regarding them as nega- 



'Biol. Bull., XVII., p. 207, 1909. 



tive, for Lillie has suggested several reasons 

 which point to the asters as regions of nega- 

 tive charge. It is on account of the prominent 

 asters present at this stage that the micro- 

 meres are free of pigment. Even when cut off 

 from the pigmented area of centrifuged eggs 

 these cells are relatively free from pigment 

 granules. 



Such small electro-negative particles, in 

 equilibrium under conditions of rest in the 

 cell, would, on an increase of permeability, 

 migrate toward the now positive cell surface. 

 A calculation (by Lillie) of the potential dif- 

 ference which might arise, based on the ob- 

 served changes in muscle cells, gives a value 

 of 14 volts per cm., which would be ample to 

 account for the changes observed in Arhacia. 

 This same movement occurs in eggs treated 

 with hypertonic sea-water and CH^COOH. 



This change in potential must be accom- 

 panied by an increase in surface tension (see 

 Lillie) and it is quite generally true that the 

 surface tension increases immediately after 

 fertilization, as indicated by the rounding up 

 of eggs which were previously oval or elon- 

 gated in shape. 



The facts which indicate an increase in 

 permeability of the surface membrane as the 

 first change taking place in the development 

 of an egg may be summarized as follows : 



1. The general similarity in the means of 

 stimulating eggs to divide and the means of 

 stimulating muscles and sensitive plants. 

 These may be broadly classified as chemical, 

 mechanical, electrical, thermal and osmotic. 



2. The fact that the chemical substances 

 which start parthenogenesis cause in other 

 cells an increase in permeability (haemolysis 

 of red blood corpuscles and loss of pigment in 

 pigment-bearing cells). 



3. Evidence that stronger concentrations of 

 development-starting substances cause loss of 

 pigment in pigmented eggs. 



4. That a secretion is the first risible change 

 occurring in many eggs. 



5. That a migration of pigment-containing 

 granules to the cell surface in Arhacia eggs 

 is caused by a region of positive charge at the 

 surface resulting from ionic interchange ac- 



