AN INTRODUCTION TO THE STUDY OF GROWTH. 23 



one, the two nuclei unite to form a single nucleus, and 

 thus modified, the fertilised ovum or egg begins to 

 grow. 



The first indication of such growth is a division of 

 the egg into a number of cells, without much enlarge- 

 ment of the total mass. 



This is apparently the arrangement of the cell 

 substance best suited for vigorous nutrition, and as the 

 increase in bulk depends on nutritive processes, the 

 importance of it is plain. 1 Later the nutritive conditions 



ccto- 



FlG. 2. Segmentation of a mammalian ovum. , b, c, semi- 

 diagrammatic X 170 diam. (Allen Thomsen, after v. Bene- 

 den, Ouain's Anatomy] ; d. Section of the ovum of the 

 rabbit during the later stages of segmentation (E. v. 

 Beneden, Ouain's Anatomy] ; ^/0=ectoderm ; ento= 

 entoderm. 



improve, and with this improvement the size of the mass 

 increases. 



The processes which lead to subdivision are first 

 evident in the nuclear structures, which become the seat 

 of complicated but, at the same time, perfectly orderly 

 changes. As a result, the constituents of the nucleus 

 divide into two portions which separate from one 

 another, the surrounding cytoplasm gathers about the 

 respective daughter-nuclei, and these two portions then 

 become marked off by the cell-membra'ne ; thus, in the 

 place of one cell, two cells are now present. By cell- 



1 Sachs, Flora, 1893. 



