48 BIOLOGICAL LECTURES. 



gastroconemius was about 0.75 per cent, to 0.85 per cent., for 

 the gastroconemius that had been tetanized from 20 to 40 

 minutes it varied from 1.2 per cent, to 1.5 per cent. 



This increase of osmotic pressure inside the muscle cell 

 leads, during normal activity, to a taking up of water from the 

 blood and lymph, and the consequence is an increase in volume. 

 The same muscle, as soon as it ceases to be exercised, begins 

 to decrease in size. Activity, therefore, plays the same role 

 in the growth of a muscle that the heat of spring plays in the 

 growth of the seed. 



Upon endeavoring to determine whether the function of seg- 

 mentation, like other functions of growth, is influenced by 

 the amount of water contained in the cell, I found that by 

 decreasing the amount of water in the ovum of the sea-urchin 

 segmentation is retarded, and that by using a sufficiently high 

 concentration of sea-water it may be stopped entirely. There- 

 fore the amount of water contained in the cell plays still 

 another role in the process of organization and influences the 

 process of cell division. 



IV. THE ARTIFICIAL PRODUCTION OF DOUBLE AND MULTIPLE 

 MONSTROSITIES IN SEA-URCHINS. 



The idea that the formation of the vertebrate embryo is a 

 function of growth has been made the basis of the embryo- 

 logical investigations of His. In a masterly way, His has 

 shown how inequality of growth determines the differentiation 

 of organs. In the blastoderm of a chick, for example, the first 

 step in the formation of the embryo is a process of folding. 

 There originates a head fold, a tail fold, a medullary groove 

 and the system of amniotic folds. According to His, all these 

 processes of folding are due simply to inequalities of growth, 

 the centre of the blastoderm growing more rapidly than the 

 periphery. It can be shown, very simply, that such a process 

 of unequal growth must, indeed, lead to the formation of exactly 

 such a system of folds as we find in the blastoderm of a chick. 

 If we take a thin, flat plate of elastic rubber, such as is used 

 for medical purposes, and lay it on a drawing-board, we can 



