194 NATURAL SCIENCE. Sept.. 1894. 



nucleus, but by the distribution of the yolk and by the shape of 

 the embryo. 



The formation of the blastula depends on several factors. 



1. The cleavage into four, eight, and sixteen causes a loosening 

 of the cells and the appearance of intercellular spaces. 



2. As the cells multiply rapidly they naturally come to be pressed 

 into an epithelial structure. 



3. As the outer cells become pressed into an epithelium, the 

 segregation of fluid and arching of the outside cause a central cavity 

 to appear and grow larger. 



Where is the room for special determinants ? The division is an 

 elementary property of all cells ; the appearance of intercellular 

 spaces is a result of cellular and external forces as the nucleus forms 

 an attraction centre round which the yolk tends to arrange itself 

 equally. 



Gastrula formation is a result of the co-operation of all the cells, 

 of the inequality in curvature, and of a series of external circum- 

 stances. To attribute this to determinants is to turn things round. It 

 is not because certain cells contain certain elements that they become 

 epithelial, digestive and so forth ; but it is because, by mechanical 

 and other reasons they come to lie in special spaces, that the corres- 

 ponding side of their character gets developed. 



Hertwig's leading objections to Weismannism are then, first, that 

 he thinks the evidence is in favour of cell-division being heirs- 

 equal division, and that, in consequence, the germ-plasm is not 

 separate from the soma, but all the cells in an organism contain the 

 cell-characters necessary to produce a complete organism. Next, a 

 large number of the characters of organisms depend upon combina- 

 tions of cells, and so cannot have determinants in a single cell. His 

 view is epigenetic in a double way. Each cell contains all the cell- 

 characters of the organism : the special characters which become 

 active are educed by its environment. Characters which depend upon 

 the co-operation of cells can come into existence only by the 

 combination of cell-characters after these have been found. In the 

 next number of Natural Science I hope to give an account of the 

 concluding portion of Hertwig's pamphlet. 



P. Chalmers Mitchell. 



