Highmore had experience with the embryo itself,
and his actual contribution as an observer of development, although
hardly epochal, is worthy of note. But despite this empirical base,
Highmore has final recourse to a hypothesis blending many ancient ideas
and substituting the Aristotelian material and efficient causes for the
"fortune and chance" he objected to in Digby's hypothesis. It was _not_
easy in the seventeenth century to avoid falling back upon some variety
of cause or force.

In 1651, about two months before publication of Highmore's _History of
Generation_, a work appeared which marks another period in
seventeenth-century English embryology. William Harvey, _De Motu Cordis_
almost a quarter of a century behind him, now published _De Generatione
Animalium_, the work he said was calculated "to throw still greater
light upon natural philosophy."[19] This book is, perhaps, not as well
known as Harvey's treatise demonstrating circulation of the blood, but
it is an important work in the history of embryology and it occupies a
prominent position in the body of English embryological literature.

In _De Generatione_, Harvey provides a thorough and quite accurate
account of the development of the chick embryo, which, in particular,
clarified that the chalazae, those twisted skeins of albumen at either
end of the yolk, were not, as generally believed, the developing embryo,
and he demonstrated that the cicatricula (blastoderm) was the point of
origin of the embryo. The famous frontispiece of the treatise shows Zeus
holding an egg, from which issue animals of various kinds. On the egg is
written _Ex ovo omnia_, a legend since transmuted to the epigram _Omne
vivum ex ovo_. The legend illustrates Harvey's principal theme, repeated
constantly throughout the text, "that all animals were in some sort
produced from eggs."[20]

If Harvey made no contribution beyond emphasizing the origin of animals
from eggs, he would deserve a prominent place in the history of
embryology. But the work is also significant in its espousal of
epigenesis, and, supported as his argument was by observation and logic,
it became the prime formulation of that concept of development during
the seventeenth and eighteenth centuries. His statement of epigenetic
development is clear:

In the egg ... there is no distinct part or prepared matter
present, from which the fetus is formed ... an animal which is
created by epigenesis attracts,