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 deyelop- 
ment is such a characteristic feature of the cmbryological 
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. 
AMeckel, in 1821, and Jater 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; véz., 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- 
