SECTION 6 

 GENERAL METABOLISM OF THE EMBRYO 



6-1. The pH of Aquatic Eggs 



It will be convenient to take first the work on the single egg-cell, 

 which has mostly been done on the eggs of marine invertebrates, 

 and then to go on to the experiments which have dealt with the 

 tissues and surrounding substances of such an embryo as that of the 

 chick. Seven principal methods have been used in studying intra- 

 cellular hydrogen ion concentration: (i) vital staining, (2) micro- 

 injection of dissolved indicators, (3) micro-injection of solid indicators, 

 (4) " micro-ecrasement " or microcompression, (5) electrometric 

 measurement by means of micro-electrodes actually in the cell, 



(6) electrometric measurement upon thawing crushed masses of eggs, 



(7) contact of indicators with crushed tissues. Each of these methods 

 suffers from certain disadvantages. A critical comparison of them 

 has been made by Reiss in his monograph on the subject, but his 

 views, which are substantially those of the Strasburg school, have 

 not been generally accepted. Vital staining is probably the least 

 valuable of all the methods, for the dye may not penetrate and 

 show a colour in the cell-interior until the cell has become com- 

 pletely abnormal^. Yet this, of course, was the general technique 

 employed by the earlier workers, who would have preferred no 

 doubt to use a pigment already naturally in the cells, as had been 

 done in other cases, if eggs containing a natural indicator could 

 have been found. The first observations were made by Schucking 

 in 1903, who on inadequate basis regarded the granules in echino- 

 derm eggs as alkaline and the protoplasm as acid. Then Loeb in 

 1906 stained sea-urchin's eggs with neutral red, expecting on theo- 

 retical grounds to observe a trend towards the acid side after fertilisa- 

 tion. This he failed to do, the dye absorbed by the eggs of Strongylo- 

 centrotus purpuratus from i/ioo molecular solution being quite red 

 both before and after fertilisation. He noticed that after remaining 

 20 minutes in ordinary sea water, the unfertilised eggs became colour- 

 less, but the fertilised ones retained their colour. Parthenogenetic 

 eggs gave the same effect. He also noted that the more development 



1 And even if it does, it may undergo chemical change at the cell-surface. 

 N E II 54 



