42 ZOOLOGY 



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 me so- 

 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. 



1. Fertilization. If we start with the mature egg and sperm 

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

 by the reduction division, one-half of the natural number of 

 chromosomes in the species. There is an increasing body of 

 evidence for the belief that the chromosomes bear the hereditary 

 characters from one cell to another and hence from one generation 

 to another. When the nucleus of the sperm unites with the 

 nucleus of the egg in fertilization one-half of the chromosomes 

 in the new nucleus thus comes from each of the parents. Fertili- 

 zation may be said then to do two things: (a) it unites the 

 substances carrying hereditary qualities from two parents (usu- 

 ally); and (6) starts the development of the egg (Figs, n, 12). 



2 . Cleavage, and the Segregation of Germ Cells. The fertilized 

 egg soon begins to divide (cleavage). The resulting cells may be 

 much alike or may be very different in size and contents, from the 

 very beginning. (See Fig. 13.) Most of these cells enter directly 

 into the making of the adult body by the differentiation into 



