FROM SIMPLE CELL TO COMPLEX ANIMAL. 37 



the mesoderm arises as a solid mass, instead of a pocket, the 

 body cavity is formed by the splitting of the mass into an 

 inner and an outer portion. When the coelom is formed by 

 several pockets the cavities of these may ultimately coalesce, 

 forming a single body cavity. Such a cavity is found in all 

 the vertebrates and in the higher invertebrates, although it 

 may become more or less obscured and modified in the adult. 



% Differentiation of Organs and Tissues. We have al- 

 ready in these three layers and their foldings the fundamental 

 outline of that differentiation which is to give us the com- 

 plex animal form found in the adult. From these layers, 

 singly or in combination, all the tissues and organs of the 

 body arise. The various layers become locally thickened, 

 folded, or otherwise modified in form by rapid cell division, 

 thus producing the beginnings of organs. At a later date 

 differentiation takes place among the cells, and tissues arise 

 (see next chapter). In general each layer gives rise to such 

 structures as its position and relation to the outer layers would 

 suggest. This is especially noticeable in the ectoderm and en- 

 toderm. The former is more closely related to the outside 

 world, and from it are produced the protective and sensory 

 structures. These include the outer portion of the skin and 

 the hard parts often associated with it, and the whole nervous 

 system together with the sensitive portions of the organs of 

 special sense. The entoderm is derived jirpm the cells which 

 contain, or at least are closely related to, the food originally 

 stored in the ovum (Fig. n), and it comes to lie in the in- 

 terior of the embryo. It furnishes the lining of the adult 

 digestive tract as well as the essential parts of the glands aris- 

 ing from it. The mesoderm gives origin to the muscles and 

 to the supportive or skeletal structures generally. Many of 

 the organs are made up of contributions from two or all of 

 these germinal layers. Students must be referred to special 

 textbooks on embryology for a more extended account of the 

 manner in which the germinal layers give rise to adult organs. 



