He THE DEVELOPMENT OF THE CHICK 
and orderly differentiation among groups of these cells. Among 
these organs are the gonads, consisting of cells which trace a 
continuous lineage by cell-division back to the fertilized ovum, 
and which are capable of developing into ova or spermatozoa 
according to the sex of the individual. 
The lives of successive generations are thus continuous because 
the series of germ-cells from which they arise shows no break in 
continuity. All other kinds of cells composing the body finally 
die. In view of this contrast the non-germinal cells of the body 
are known collectively as somatic cells. In some way the germ- 
cells of a species maintain very constant properties from gen- 
eration to generation in spite of their enormous multiplication, 
and this furnishes the basis for hereditary resemblance. 
The establishment of the fact that in all animals the ovum is 
a single cell, and that the cells of all tissues of the body are derived 
from it by a continuous process of cell-division, completes the 
outline of the cycle of the generations, and furnishes the basis 
for a complete theory of development. The full significance 
of this principle can only be appreciated by learning the condition 
of embryology before the establishment of the cell-theory in the 
eighteenth century. The history of our knowledge of the devel- 
opment of mammals is particularly instructive in this respect: 
some knowledge had been gained of the anatomy of the embryos, 
mostly relatively advanced, of a few mammals; but the origin 
of the embryo was entirely unknown; the ovum itself had not 
been discovered; the process of fertilization was not understood. 
In the knowledge of the cycle of generations there was a great 
gap, and the embryo was as much a mystery as if it had arisen 
by a direct act of creation. To be sure Harvey in 1651 had 
propounded the theorem, omne vivum ex ovo, but no one had 
ever seen the egg of a mammal, and there was no clear idea in 
the case of other forms what the ege@ signified. 
In 1672, de Graaf (who died in 1673 at the age of 32) published 
a work, “de mulierum organis generationis inservientibus,”’ in 
which he attempted to show that the vesicles seen on the surface 
of the ovaries were the female reproductive material. But he 
could not reconcile the view that the Graafian follicle is the mam- 
malian egg with the fact that the earllest embryos discovered 
by him were smaller than the follicles. For this reason his views 
were opposed by Leeuwenhoek and Valisnieri; and the later re- 
