GROWTH AND DIFFERENTIATION OF THE EMBRYO 5 



and especially the muscles gain ; the pancreas, digestive tube, and lungs are 

 little affected. 



Continuity of the Germ Plasm. According to this important concep- 

 tion of Weismann, the body-protoplasm, or soma, and the reproductive- 

 protoplasm differ fundamentally. The germinal material is a legacy 

 that has existed since the beginning of life, from which representative 

 portions are passed on intact from one generation to the next. Around 

 this germ plasm there develops in each successive generation a short-lived 

 body, or soma, which serves as a vehicle for insuring the transmission and 

 perpetuation of the former. The reason, therefore, why offspring resem- 

 bles parent 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 develop- 

 ment repeats hastily and incompletely the evolutionary history of its own 

 species. This law of recapitulation was first stated clearly by Miiller 

 in 1863 and was termed by Haeckel the law of biogenesis. In accordance 

 with it, the fertilized ovum is compared to a unicellular organism like the 

 Amoeba; the blastula is supposed to represent an adult Volvox type; 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. More- 

 over, 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, and the arrange- 

 ment of blood vessels is equally primitive; 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. 



Some apparently useless structures appear during development, 

 perfunctorily reminiscent of ancestral conditions; certain other parts, of 

 use to the embryo alone, are later replaced by better adapted, permanent 

 organs. Representatives of either of these types may eventually dis- 

 appear or they may persist throughout life as rudimentary organs; more 

 than a hundred of the latter have been listed for man. Still other ancestral 

 organs abandon their provisional embryonic function, yet are retained in 

 the adult and utilized for new purposes. 



Methods of Study. Human embryos not being available for indi- 

 vidual laboratory work, the embryos of the lower animals which best illus- 

 trate 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; early stages of echinoderms, or of worms, are 

 frequently used to demonstrate the cleavage of the ovum and the develop- 

 ment of the blastula and gastrula; the chick embryo affords convenient 

 material for the study of the early vertebrate embryo and the formation of 



