COMPARATIVE EMBRYOLOGY 799 



would probably not have had quite the place in the history of embry- 

 ology that it does if the cell doctrine had not developed before it. 

 The cell doctrine, coming in 1838-1839, made it possible to carry 

 knowledge of the organization of the embryo back beyond the stages 

 of germ-layer formation and it gave the basis from which an under- 

 standing of the formation of tissues and organs could proceed. The 

 ceU theory, the improvements on the microscope, and the develop- 

 ment of new methods of fixing, sectioning, and staining material made 

 possible the accumulation of a great deal of information about the 

 origin and the details of development of the embryos of all kinds of 

 animals, and this was a special contribution of the last third of the 

 nineteenth century. The twentieth century has extended this body 

 of knowledge and interpreted it upon the basis of extensive experi- 

 mentation. 



For convenience, the developmental history of an animal may be 

 divided into stages, but the student must remember that this is an 

 arbitrary action on our part, and that the development from the ani- 

 mal's standpoint is a continuous process. It must be borne in mind 

 that ontogeny, which means the entire life history of an organism 

 from the earliest beginnings to old age and death, is a continuous 

 and ever-changing set of processes, and the sum of the characteristics 

 of all the stages of development makes up the characteristics of the 

 animal. The hen's egg is as much a representative of the genus and 

 species, Gallus domesticus, as is the feathered animal to which most 

 of us attach that name. 



Embryology really begins with the formation of the germ cells in 

 the bodies of the parents, and it is known in some forms that the 

 prospective germ cells are set aside very early in the development of 

 the organism, even in the cleavage stages, and that they may remain 

 latent until the organism has become independent in its existence. 

 The development of the germ cells, or gametes, through the active 

 stages of gametogenesis (Fig. 47) involves the multiplication period 

 during which they are known as "gonia" {oogonia and spermato- 

 go7iia), the growth period, the maturation period proper, that is, the 

 period of the "cytes" (oocytes and spermatocytes), ending with the 

 fully formed or mature oogonia and spermatogonia. These gametes 

 are now ready for the process of fertilization, which takes the form of 

 two steps, the first the initiation of development on the part of the 

 egg following the entrance of the sperm ; and, second, the union of 



