SOME GENERAL CONCEPTS IN EMBRYOLOGY 13 



any germ cell. Nevertheless, one can bring together in the same re- 

 ceptacle the fertilized eggs of closely and distantly related forms and 

 yet their individual development is unaffected. A normally fertilized 

 egg of the starfish will always develop into a starfish, a frog's egg into 

 a frog, a guinea pig egg into a guinea pig. The environment of the 

 fertilized egg has never been known to cause the transmutation of 

 species. Further, we cannot see the stripes on a 2-cell mackerel egg, 

 or the green and yellow spots on the 2-cell frog's egg, or the colorful 

 plumage on a peacock blastoderm, or the brown eyes of the human 

 optic vesicle. Nevertheless these intrinsic genetic potentialities are defi- 

 nitely preformed and inevitable in development under normal en- 

 vironmental circumstances. Development is epigenetic to the extent 

 that one cannot see the formed structures within the egg, or the sperm, 

 or zygote. However, the organism is preformed chemically (geneti- 

 cally) to the extent that its specific type of development is inevitable 

 under a given set of circumstances. Our modern interpretation is there- 

 fore intermediate between that of preformationism and epigenesis. 

 There is unfolding development within certain preformed limits which 

 must be chemical and/or physical, but not visibly morphological. 



Soma and Germ Plasm 



It is definite that in some forms there is an early segregation of 

 embryonic cells so that some will give rise to somatic (body) tissues 

 while others will give rise to germ (reproductive) tissues. This was 

 the contention of Weismann and many others about 1900. The germi- 

 nal cells seem to come from the extra-embryonic regions of many 

 vertebrates and become differentiated at an early stage of develop- 

 ment. Whether or not these migrating pre-germ cells are the precursors 

 of the functional gametes has not been determined conclusively. 

 Nevertheless, the germ plasm, once segregated, is immune to damage 

 by the somatoplasm. This has been illustrated many times by men 

 who have incurred injuries and yet who have subsequently produced 

 normal offspring. The germ plasm is therefore functionally isolated, 

 segregated in the adult. Further, x-ray damage to the germ plasm does 

 not show up in the somatoplasm at least until the next generation. 



Germ plasm can and does give rise to somatoplasm during normal 

 embryonic development. There is little, if any, evidence that somato- 

 plasm, once formed, can give rise to germ plasm. When the fully 

 formed gonads of higher animals are removed in their entirety, there 



