METHODS OF STUDY 5 



fore, why offspring resembles its parents is because each develops from portions 

 of the same stuff. 



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

 stantly observed in studying embryos, that the individual in its development 

 tends to repeat the evolutionary history of its own species. This law of recapitu- 

 lation was first stated clearly by Miiller in 1863 and was termed by Haeckel the 

 law of biogenesis. According to this law, the fertilized ovum is compared to a 

 unicellular organism like the Amoeba; the blastula 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-hke 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, Kke that of the fish; the kidney of the embryo is like that of the amphib- 

 ian, as are also the genital ducts. Many other examples of this law may readily 

 be observed. 



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

 laboratory work, the embryos of the lower animals which best illustrate certain 

 points are employed instead. Thus the germ cells of Ascaris, a parasitic round 

 worm, are used to demonstrate the phenomena of mitosis and maturation; the 

 larvae of echinoderms, or of worms, are frequently used to demonstrate the cleav- 

 age of the ovum and the development of the blastula and gastrula larvae; the 

 chick embryo affords convenient material for the study of the early vertebrate em- 

 bryo, of the formation of the germ layers and of the embryonic membranes, while 

 the structure of a mammahan embryo, similar to that of the human embryo, is 

 best observed in the readily procured embryos of the pig. An idea of the anatomy 

 of embryos is obtained first by examining the exterior of whole embryos and study- 

 ing 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 

 the student is referred. Below are given the titles of some of the more iniportant 

 works on vertebrate and human embryology, to which the student is referred 

 and in which supplementary reading is recommended. 



