REPRODUCTION AND DEVELOPMENT IN CHORDATES 



The embryo is a living individual, potentially self-sufficient at every stage in 

 development, yet dependent, as is the adult, upon external conditions for sur- 

 vival. Many problems of development remain to be solved, and many new 

 methods of research have been devised in the attack upon the unknown in 

 this field of zoology. It is a fascinating and a rich field for further study. 



Summary 



This completes your study, in the vertebrate animal, of the manifestations 

 of life deriving from the distinguishing capacities of the cell, namely metab- 

 olism, responsiveness, and reproduction. You have seen how the difTerent 

 kinds of mutually dependent, specialized somatic cells perform a myriad of 

 adaptive functions concerned with the maintenance of the life of the multi- 

 cellular individual. Even in the embryo, as it develops from a single cell, 

 this closely adjusted relationship between different cells and between cells and 

 environment is maintained. Elucidation of the complexity of structure and 

 function has gone forward for many years with the help of the Cell Theory, 

 although at the present time, great contributions are being made by studies 

 at the subcellular level. 



In spite of the close attention to detail which is required to grasp the many 

 facts presented, it is hoped that you have been stimulated to think beyond 

 the details, even beyond the major aspects of how the problems of living have 

 been solved at the cellular level. What really makes these intricate mech- 

 anisms go in such orderly ways? And how does it happen that the basic func- 



Fig. 5.31. Embryos of Rana 

 pipiens treated briefly with 

 ailcali during tiie early gas- 

 trula stage; both are the same 

 age. ,-1 has only one eye in 

 the median position; B is 

 partially doubled in the region 

 of the spinal cord and tail. 

 (Redrawn from Analysis of 

 Development, edited by B. H. 

 Willier, P. A. Weiss, and V. 

 Hamburger, copyright 1955 

 by W. B. Saunders Co., 

 printed by permission.) 



External gills 

 Median eye 

 Sucker 



Mouth 



169 



