GROWTH 



305 



Limbs arise, a skeleton is formed, the blood-vessels make 

 their appearance, and with growth changes that give the 

 body its final shape the skin gives rise to the scales, 

 feathers or hair that are characteristic of the adult. 



Growth. — In the development of an animal from the 

 egg to the adult the body absorbs yolk and other food 

 substances, forms protoplasm in excess of that which is 

 destroyed, and thus grows. In early life, especially, the 

 increase in bulk is very rapid but it gradually slows down' 

 as the adult condition is approached. During this period 



Fig. 73. — Development of Frog. A, early development of central 

 nervous system of frog. B, closing of nervous system folds. C, later 

 state with skin cut away to show complete separation of nervous 

 system and a simple skeleton. Three gill slits are shown near mouth 

 opening. 



the animal may be starved and remain small, becoming a 

 dwarf, or it may be abundantly nourished and grow to 

 large size. Nevertheless, growth has its limits, except 

 perhaps in the case of certain sponges and a few species 

 of devilfishes. A mouse, for example, may be supplied 

 with an unlimited amount of food yet it never grows to 

 the size of a rat, though the bodily organization is 

 practically the same in both. 



Differentiation. — Development proceeds, as has 

 been described, from a stage where no organs exist 

 through a stage where they are but roughly outlined and 

 only gradually reaches the point where they are in a 

 completed functional condition. If the cells are ex- 

 amined during this period they also will be seen to pro- 



