FORM, GROWTH, AND CHANGE 31 



these, we must pass to the outline of those processes by which 

 the body of the young insect is built up within the egg. 



The development of any living creature from an egg is 

 brought about by successive cell-divisions, so that the multitu- 

 dinous cells which build up the body of the new animal may 

 be regarded as descendants of the original egg-cell. We have 

 seen that in an insect's egg the living protoplasm surrounds, for 

 the most part, a central yolk. Consequently the preliminary 

 cell-divisions leading to what is known as the segmentation of 

 the egg take place in the active external protoplasmic region, 



FIG. 17. EMBRYO OF WATER-BEETLE (HydrOphilus). 



Ventral view showing developing segmentation of body with 

 appendages, and amnion. x 30. From Comstock after 

 Heider. 



resulting in the formation of an enveloping sheet of cells (the 

 blastoderm) around the central mass of food-yolk. 



Along one face of the segmenting egg the blastoderm 

 becomes thickened forming the germ-band, the position of 

 which marks the ventral region of the embryo. In the germ- 

 band,the cells become differentiated into an outer and an inner 

 layer. The outer layer (or ectoderm) gives rise to the skin (or 

 epidermis), the nervous system, the linings of the air-tubes 

 and of the fore- and hind-guts as already mentioned. From 

 the inner layer are derived all the other organs of the body 



