14 INTRODUCTION 



Throughout life, tissue cells are undergoing retrogressive changes. In this 

 way the cells of certain organs like the thymus gland and mesonephros degenerate 

 and largely disappear. The cells of the hairs and the surface layer of the epider- 

 mis become cornified and eventually are shed. Tissue cells may thus normally 

 constantly be destroyed and replaced by new cells. 



The Law of Biogenesis. — Of great theoretical interest is the fact, con- 

 stantly observed in studying embryos, that the individual in its develop- 

 ment recapitulates the evolution of the race. This law of recapitulation was 

 asserted by Meckel in 1881 and was termed by Haeckel the law of biogenesis. 

 According to this law, the fertilized ovum is compared to a unicellular organism 

 like the amceba; the blastula embryo is supposed to represent an adult Volvox; 

 the gastrula, a simple sponge; the segmented embryo a worm- like stage, and the 

 embryo with gill-slits may be regarded as a fish-like stage. The blood of the 

 human embryo in development passes through stages in which its corpuscles 

 resemble in structure those of the fish and reptile; the heart is at first tubular, like 

 that of the fish; the kidney of the embryo is like that of the amphibian, as are 

 also the genital ducts. Many other examples of this law may readily be observed. 

 A more complete account of the general conceptions of embryology is given in 

 Minot's " Laboratory Textbook of Embryology." 



Methods of Study. — Human embryos not being available for individual 

 laboratory work, we employ instead the embryos of the lower animals which 

 best illustrate certain points. Thus the ova of Ascaris, a parasitic round worm, 

 are used to demonstrate the phenomena of mitosis; the larva? of echinoderms, 

 or of worms, are frequently used to demonstrate the segmentation of the ovum 

 and the development of the blastula and gastrula larva?; the chick embryo af- 

 fords convenient material for the study of the early vertebrate embryo, of the 

 formation of the germ-layers and of the embryonic membranes, while the struc- 

 ture of a mammalian embryo, similar to that of the human embryo, is best ob- 

 served in the embryos of the pig, which are very readily obtained. An idea of 

 the anatomy of the embryos is obtained first by examining the exterior of whole 

 embryos and studying dissections and reconstructions of them. Finally, each 

 embryo is studied in serial sections, the level of each section being determined by 

 comparing it with figures of the whole embryo. 



Along with his study of the embryos in the laboratory, the student should 

 do a certain amount of supplementary reading. Only the gist of human organo- 

 genesis is contained in the following chapters. A very complete bibliography 

 of the subject is given in Keibel and Mall's "Human Embryology," to which 



