680 The Outline of Science 



Every higher animal starts life as a single cell, the fertilized 

 ovum in the case of man ; this measures no more than T i 5 of an inch 

 in diameter. The processes of development can all be thought 

 of in terms of cells, their multiplication, their migration, and their 

 changes of form. 



The first stages in development consist in the cutting up of 

 the ovum into a number of similar or almost similar rounded cells. 

 In the next stage, the rough ground plan of the future embryo 

 is blocked out by the formation of the three layers of cells to be 

 found in all higher animals. 



During the succeeding stage, a more detailed plan is sub- 

 stituted for the rough sketch, and the main organ-systems are 

 laid down. The outer of the three layers gives rise to the future 

 brain and spinal cord, the eye, ear, and nose, the outer skin; the 

 innermost layer modifies itself into a sketch of the gut, with liver, 

 sweetbread, thyroid and other glands; while the intermediate 

 layer produces the rudiments of the blood-system, the kidneys, 

 the muscles, and the skeleton. With the intermediate layer 

 are also associated the reproductive cells, but they, as we have 

 seen, stand somewhat apart from the ordinary tissues of the 

 body. 



We have spoken of the laying-down of a plan. That and no 

 more is what development has up to this stage produced; the 

 rudiments of the future organs are distinguishable, they are in 

 their proper places, but they do not yet work; and they do not 

 work because the cells of which they are composed are still 

 all nearly alike and not yet specialised to perform different 

 duties. 



It is only in the succeeding stage, the stage of tissue-differ- 

 entiation, that the various types of cell lose their original rounded 

 or cubical forms, and begin assuming the appearance which they 

 will have in the developed organism. 



Some of the chief types of cell-change that occur may be 

 briefly mentioned. 



