14 INTRODUCTION 



seems to be no regeneration of germinal tissues from the remaining 

 somatic tissues. 



However, modern embryologists are rather reluctant to believe that 

 these two types of protoplasm are as fundamentally segregated from 

 each other as was once believed. Regeneration is a characteristic of 

 lower forms. In these cases it certainly involves a redevelopment of 

 germinal tissue (e.g., Planaria regeneration of a total organism from 

 one-sixth part completely devoid of germinal tissue). The lack of 

 regenerative powers in general among the higher forms may be the 

 intervening factor in the distinction between somatoplasm and germ 

 plasm, and the lack of interchange or regeneration between them. 



The Germ Layer Concept 



In 1817 Pander first identified the three primary germ layers in the 

 chick embryo, and since then all metazoa above the Coelenterata have 

 been proved to be triploblastic, or tri-dermic. The order of develop- 

 ment is always ectoderm, endoderm, and then mesoderm, so that the 

 most advanced forms in the phylogenetic scale are those possessing 

 mesoderm. 



We tend to forget that these germ layer distinctions are for human 

 convenience and that the morphogenetic potentialities are relatively 

 unaware of such distinctions. One can exchange presumptive regions 

 of the blastula so that areas normally destined to become mesoderm 

 may remain in a superficial position to function as ectoderm, or be- 

 come endoderm. The exchanges may be made in any direction in the 

 early stages. The embryo as a whole may develop perfectly normally, 

 with exchanged presumptive germ layer areas. 



When we remember that all tissues arise from cells having the 

 same origin (i.e., from the zygote), and that mitosis ensures similar 

 qualitative and quantitative inheritance, then the presumed distinc- 

 tions of the three primary germ layers seem to dissolve. It is only dur- 

 ing the later phases of development (i.e., during differentiation) that 

 the totipotent genetic capabilities of the cells become delimited, and we 

 have the appearance of structurally and functionally different cell 

 and tissue types. 



The Normal Sequence of Events in Embryology 



Cell Multiplication. 



From the single cell (the fertilized egg or zygote) are derived the 

 many millions of cells which comprise the organism. This is done by 



