THE RISE OF EMBRYOLOGY 229 



mental processes in the light of the hypothesis of organic 

 evolution. His speculations were sufficiently reserved, and 

 nearly always luminous. It is significant of the character 

 of this work to say that the speculations contained in the 

 papers of the rank and file of embryological workers for more 

 than two decades, and often fondly believed to be novel, 

 were for the most part anticipated by Balfour, and were also 

 better expressed, with better qualifications. 



The reading of ancestral history in the stages of develop- 

 ment is such a characteristic feature of the embryological 

 work of Balfour's period that some observations concerning 

 it will now be in place. 



Interpretation of the Embryological Record. — Perhaps 

 the most impressive feature of animal development is the 

 series of similar changes through which all pass in the embryo. 

 The higher animals, especially, exhibit all stages of organiza- 

 tion from the unicellular fertilized ovum to the fully formed 

 animal so far removed from it. The intermediate changes 

 constitute a long record, the possibility of interpreting which 

 has been a stimulus to its careful examination. 



Meckel, in 1821, and later Von Bacr, indicated the close 

 similarity between embryonic stages of widely different 

 animals ; Von Baer, indeed, confessed that he was unable to 

 distinguish positively between a reptile, a bird, and a mam- 

 malian embryo in certain early stages of growth. 



In addition to this similarity, which is a constant feature of 

 the embryological record, there is another one that may be 

 equally significant ; viz., in the course of embryonic history, 

 sets of rudimentary organs arise and disappear. Rudiment- 

 ary teeth make their appearance in the embryo of the whale- 

 bone whale, but they are transitory and soon disappear with- 

 out having been of service to the animal. In the embryos 

 of all higher vertebrates, as is well known, gill-clefts and 

 gill-arches with an appropriate circulation, make their ap- 



