14 INTRODUCTORY I 



When segmentation is concluded the blastula stage is 

 reached, with a segmentation cavity. Into this pass the 

 mesoderm cells, while the endoderm is invaginated to form 

 the gut. The blastopore is shifted to the anterior end, where 

 it is encircled by the stomodaeal cells. 



The cell-lineage which we have just described may be 

 displayed in diagrammatic form thus : the cells in which the 

 chromosomes remain intact being indicated by a cross. 



ovum 



P 2 S 2 (E M St.) 

 PS S 3 ?C) E 

 S 4 (D) 



The egg of A scar is has been made the subject of important 

 experiments by Boveri. 



And now let us turn to our main thesis, the inquiry into the 

 causes of growth, cell-division, and differentiation. 



We shall begin with growth. 



Growth is increase in size, and may be measured by increase 

 in weight, sometimes by increase in other dimensions, such as 

 stature, girth, and so on. Growth is partly the increase of 

 living material, in part the increase of substances secreted by 

 that living material. These secretions may be intra-cellular, 

 for instance the contractile substance of a muscle fibre, or 

 extra-cellular, as the chitinous cuticle of an Arthropod ; they 

 may be organic, as in the examples quoted, or inorganic, such 

 as sponge spicules or the calcareous matrix of bone. Growth 

 obviously depends upon the assimilation of food-material, 

 which in early stages is usually provided by the yolk. It 



