Embryology 279 



tures. It was obviously more than a simple vmfolding and growth in 

 size. This progressive development of more complex structures from 

 simple beginnings is known as the theory of epigenesis. 



With the discovery of the genes and their importance as carriers of 

 hereditary units, our present theory of the mode of development of the 

 individual is in a sense a compromise between the preformists and the 

 believers in epigenesis. It is now realized that the genes or hereditary 

 units are passed from generation to generation, and are thus really pre- 

 formed units which to a great extent determine the course of develop- 

 ment. They are, however, the controllers of the rate and pattern of de- 

 velopment while the development itself occurs through a series of com- 

 plex changes and reorganizations. This latter, of course, is reminiscent 

 of epigenesis. 



Even after preformation was shown to be impossible, scientists still 

 realized that something was passed on from generation to generation, and 

 whatever it was, it must be contained within either the egg or sperm or 

 perhaps in both. Different investigators tended to emphasize dift'erent 

 methods in which these "like qualities" were passed on, but Weismann 

 (1883) was the first to formulate the theory which is now believed to be 

 essentially true. He argued that the sex cells were passed on from gen- 

 eration to generation in an unbroken line; that somatic or body cells 

 were only the guardians of these precious cells. The somatic cells were 

 produced anew in each individual, but were essentially transitory. This 

 is the theory of the continuity of the germplasm. With the discovery 

 of the gene and its hereditary control, it became clear that it was the 

 genes which were passed on from generation to generation. In this way, 

 Weismann thus was shown to be essentially correct in his regard of the 

 importance of the germplasm. 



In the embryology of the individual, certain definite stages can be 

 recognized. Clearly, this is a continuous process, and any divisions are 

 simply arbitrary ones with rather vague beginnings and endings. Those 

 first stages which precede organ formation are known as early embry- 

 ology. These begin with the first divisions of the zygote and end ap- 

 proximately when the organs begin to form. At the end of this first 

 period, the embryo has attained its three basic layers of embryonic tissue 

 known as germ layers. The second period is concerned with the de- 

 velopment of organs and organ systems from these basic germ layers. 

 This is the period of organogeny. 



