CHAPTER XI. 



THE LATER DIFFERENTIATION OF THE ANIMAL 

 BODY. 



121. A Resume of the Early Stages. The new individual may 

 be said to enter upon its life when the ovum is fertilized. The cell 

 divides, and these daughters divide, and we have successively a 

 2, 4, 8, 1 6, 32-celled mass (morula, Fig. 20). The cell divisions 

 continue, and by the time the food in the egg is used up the young 

 animal must be far enough along to use food from the outside. This 

 results in growth and further cell divisions. Sooner or later, by these 

 means, a two-layered sac of cells (gastrula, Fig. 20, 4), is formed, 

 made up of ectoderm and entoderm. These two layers of cells have 

 very different fate in the later development of the individual. 



122. Mesoderm. In addition to these two primitive layers there 

 is another mass of cells which comes to lie between the ectoderm and 

 entoderm. It is somewhat less definite than the other layers and 

 may be formed in several different ways. It is usually derived from 

 the entoderm or the ectoderm or both. This mass of cells is known 

 as mesenchyme or mesoderm. The mesoderm may entirely fill the 

 cavity between ectoderm and entoderm, or it may split so as to 

 contain a cavity completely surrounded by mesoderm. Such a 

 cavity is known as the calom, or body cavity. It will be seen from 

 Fig. 21 that this cavity surrounds the digestive tract. Within it 

 are developed and located most of the internal organs. 



123. Organs Derived from the Three Germinal Layers. From these 

 three layers of cells all the organs of the body arise. 



From the ectoderm come the outer layers of the skin together 

 with its outgrowths, such as hair, feathers, and nails; the nervous 

 system, including the brain and central nervous organs; and the 

 sensitive cells, such as the retina of the eye, the taste cells, the 

 epithelium of the inner ear, and the like. 



The entoderm furnishes the lining of the digestive tract and of 

 the glands that spring from it, including the lungs of vertebrates. 



The mesoderm produces the muscles, the inner layers of the skin, 



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