2 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 egg 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 contained the female reproductive material in 

 bladder-like form. But he could not reconcile this view of the 

 Graafian follicle with the fact that the earliest embryos discovered 

 by him were smaller than the follicles. For this reason his views 

 were opposed by Leeuwenhoek and Valisnieri; and the later re- 



