418 DEVELOPMENT OF FROG AND OTHER VERTEBRATA 



When less was known of the details of embryology, it was thought 

 that development consisted in the appearance of the germ layers as 

 the earhest recognizable differentiation of the embryo, and in the 

 subsequent formation of adult parts from one or another of these 

 three types of cells. It was then believed that the differentiation 

 into ectoderm, endoderm, and mesoderm was irrevocable; and any 

 case in which one germ layer gave rise to cells of another was 

 held to be very exceptional. In its general features, this " germ- 

 layer theory " can still be accepted as a description of such steps 

 in differentiation as are indicated by the foregoing table. But 

 there are many exceptions, as where certain cells of a regenerating 

 animal form tissues that originate from all three of the germ 

 layers during embryonic development. Moreover, the germ 

 layers have themselves been traced to an earlier origin in the 

 studies of cell lineage by which they have been followed back 

 through all the preceding cell divisions to the zygote. In some 

 animals it is also found that the principal regions of the adult 

 body may be recognized in the cytoplasm of the zygote after the 

 manner described for the frog, and hence there is a promorphology 

 in the one-cell stage which is a first step in differentiation, even 

 before cell division has begun. 



Remarkable as it may seem, the development of some animals 

 is, therefore, traceable not only to single cells or groups of cells 

 that form germ layers and later organs, but even to the cytoplasm 

 of the zygote. The germ cells, however, reveal no such organiza- 

 tion and hence must be regarded as having the capacity to produce 

 the organization seen in a zygote, which then proceeds with cell- 

 division and differentiation, constituting a process of epigenesis, or 

 the formation, step by step, of parts that gradually assume their 

 resemblance to adult structures. This process should be dis- 

 tinguished from the preformation, which was supposed to exist 

 by some eighteenth century embryologists, who represented the 

 human spermatozoon as containing a miniature human being, the 

 homunculus, which developed to an adult by a mere process of 

 growth. It is now known that development, in its general features, 

 is by epigenesis from germ cells with no exact resemblance to 

 the adult, and not by the unfolding of a miniature organism that 

 is preformed within the ovum or spermatozoon. The significance 

 of the statement that development is a process of cell division and 

 differentiation is thus concretely illustrated. The cell division is 



