176 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
relationship. None of them denies and many strongly affirm that 
embryology affords some of the strongest and most convincing evi- 
dence in favor of the evolutionary theory. 
Let us examine some of this evidence. To begin with, it should 
be noted that, in following out the ontogeny or individual develop- 
ment, the observer witnesses the formation of something new, not 
merely the enlargement and unfolding of a pre-existing organism, 
though the theory of preformation, which was widely accepted in the 
eighteenth century, looked upon ontogeny precisely in that way, as 
the growth of a germ which was the miniature of the parent. Sucha 
theory was possible only before the development of microscopic 
technique had enabled the observer to detect the actual successive 
steps of change. The egg is a single cell, with the nucleus and all the 
parts of other undifferentiated cells, though it may be enormously 
enlarged by the presence of food-yolk. In the hen’s egg this food-yolk 
is quite inert and the activity of development is confined to the minute 
disc of protoplasm on the outside of the yolk, while in the frog’s egg 
the yolk is disseminated, though not uniformly, throughout the egg 
and in the mammalian egg, which is microscopic in size, there is no 
yolk. Itis.a very remarkable fact that all of the vertebrated animals, 
fishes, amphibians, reptiles, birds and mammals, however different 
their habits and modes of life, have a mode of ontogeny which is of 
even more characteristically and unmistakably the same plan than is 
the type of their adult structure, which was described in the last 
chapter. The egg, or the active portion of it, divides in a definite and 
regular manner into a very large number of cells, which arrange them- 
selves in definite layers, an outer and an inner, and within these layers 
-cell-aggregates form incipient organs, which, step by step, take on the 
adult condition. Not only is the plan and type of development 
essentially similar throughout the whole phylum of the vertebrates, 
but, in accordance with the recapitulation theory, many structural 
features which are permanent in lower forms appear in the embryos of 
higher and more advanced types. In the latter, however, these 
features are transitory and, in the course of development, they either 
disappear, or are so modified as to be very different, sometimes unrecog- 
nizable, in the adults. 
At a certain stage of the ontogeny the embryo of a mammal has 
gill-pouches like a fish, the skeletal supports of the gill-pouches, the 
arteries and veins which supply them with blood, the structure of the 
heart, in short, the entire plan of the circulatory system is fish-like. 
