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DARWINISM 
CHAP. 
The Embryonic Development of Man and other Mammalia. 
The progressive development of any vertebrate from the 
ovum or minute embryonic egg affords one of the most 
marvellous chapters in Natural History. We see the con¬ 
tents of the ovum undergoing numerous definite changes, 
its interior dividing and subdividing till it consists of a 
mass of cells, then a groove appears marking out the median 
line or vertebral column of the future animal, and there¬ 
after are slowly developed the various essential organs of 
the body. After describing in some detail what takes place 
in the case of the ovum of the dog, Professor Huxley 
continues: “ The history of the development of any other 
vertebrate animal, lizard, snake, frog, or fish tells the same 
story. There is always to begin with, an egg having the 
same essential structure as that of the dog; the yelk of 
that egg undergoes division or segmentation, as it is called, 
the ultimate products of that segmentation constitute the 
building materials for the body of the young animal; and 
this is built up round a primitive groove, in the floor of which 
a notochord is developed. Furthermore, there is a period in 
which the young of all these animals resemble one another, 
not merely in outward form, but in all essentials of structure, 
so closely, that the differences between them are inconsider¬ 
able, while in their subsequent course they diverge more and 
more widely from one another. And it is a general law that 
the more closely any animals resemble one another in adult 
structure, the larger and the more intimately do their embryos 
resemble one another; so that, for example, the embryos of a 
snake and of a lizard remain like one another longer than do 
those of a snake and a bird ; and the embryos of a dog and 
of a cat remain like one another for a far longer period than 
do those of a dog and a bird, or of a dog and an opossum, or 
even than those of a dog and a monkey.” 1 
We thus see that the study of development affords a test 
of affinity in animals that are externally very much unlike 
each other; and we naturally ask how this applies to man. 
Is he developed in a different way from other mammals, as 
we should certainly expect if he has had a distinct and 
1 Man's Place in Nature , p. 64. 
