12 INTRODUCTION 



of embryos of various forms. He never suggested that embryos of 

 higher forms recapitulated the adult stages of their ancestors; that man 

 as an embryo went through such stages as those of the adult fish, 

 amphibian, reptile, bird, and finally anthropoids and man. The simi- 

 larities referred to were in the early developmental stages, and most 

 definitely not in the adults. 



As one studies comparative embryology it becomes clear that there 

 is a basic similarity in development, and that the brain, nerves, aortic 

 arches, and metameric kidney units (for instance) develop in a some- 

 what similar manner from the fish to man. Proponents of the theory 

 of evolution have emphasized this as evidence in support of their 

 theory. It is, however, possible that this similarity is due to a highly 

 selective environment which has eliminated those types of develop- 

 ment which have digressed from a certain basic pattern. Put another 

 way, the type of development we now know was suited to survive in 

 the environments available. Three primary germ layers may have 

 proved to be more efficient (i.e., to have better survival value) than 

 two or four, and so the gradual transition from pro- through meso- 

 and finally to the meta-nephros may be a necessary corollary of the 

 slow developmental process. Von Baer's laws may simply represent 

 one method of expressing a fundamental law of nature (i.e., survival 

 values) rather than a phylogenetic relationship. 



There are many specific instances in development which could be 

 used to refute the Law of Recapitulation as it is often stated, but which 

 would in no way refute von Baer's "Biogenetic Law." One example 

 may be cited: there is no evidence among the lower forms of any antici- 

 pation of the development of the amnion and the chorion of the 

 amniotes. The Biogenetic Law was derived from a study of compara- 

 tive embryology, and possibly it has significance in the understanding 

 of the mechanism of evolution. 



Epigenesis vs. Preformationism 



The historical sequence from preformationists to spermists and 

 then ovists was very natural. The refinement of optical equipment 

 dispelled these earlier concepts and the pendulum swung to the opposite 

 extreme where embryologists believed that nothing was preformed 

 and that development was entirely epigenetic. As often occurs, the 

 pendulum has swung back to an intermediate position today. 



No one has ever seen any preformed structure of the organism in 



