42 ZOOLOGY 



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. 



59. 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 complex 

 animal form found in the adult. Prom 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 begin- 

 nings 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 other layers would suggest. This is especially noticeable 

 in the ectoderm and entoderm. The ectoderm is more closely 

 related to the outside world, and from it are produced the pro- 

 tective 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, as the retina of the eye. The entoderm 

 is derived from the cells which contain, or at least are closely 

 related to, the food originally stored in the ovum (Fig. 13), and 

 it comes to lie in the interior of the embryo. It furnishes the 

 lining of the adult digestive tract as well as the essential parts 

 of the glands and other outgrowths arising from it. The meso- 

 derm gives origin to the muscles and to the supportive tissues, 

 to the blood vessels and blood. 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. 



60. Summary of the Life-cycle and the Meaning of the 

 Steps. — It is important that the student bring together the 

 essential points in the cycle of events that occur in the life 

 history of an organism from one generation to the next. 



I. Fertilization. — If we start with the mature egg and sperm 

 ready to unite, we must recall (§50) that each of these has lost. 



