274 ANIMAL BIOLOGY 



membranes. These contribute to the formation of the placenta 

 and are modified and diverted to meet new conditions demanded 

 by the uterine life of the Mammalian embryo. (Figs. 133, 134.) 



With merely this outline of some of the chief features of the 

 embryology of the Earthworm, Frog, and higher forms before us, 

 it is possible to gain some appreciation of the similarity of the 

 basic method of development which is ever present in the Animal 

 Kingdom — cleavage, blastula, gastrula, primary germ layers, etc. 

 In general, it may be said that in all the higher animals the ecto- 

 derm forms the outer skin and nervous system ; the endoderm sup- 

 plies the lining membrane of the major part of the alimentary tract; 

 while the mesoderm contributes muscles, blood vessels, reproduc- 

 tive organs, and the membrane lining the coelom. This similarity 

 in origin of the organ systems from the primary germ layers, 

 throughout the animal series above the Coelenterates, is of the 

 highest significance because it indicates a fundamental structural 

 similarity in the body plan of all these forms. It is exhibited in 

 the developmental process in each generation, even though the 

 adult body in the various groups differs widely in form and arrange- 

 ment of organs. Such a state of affairs clearly suggests an heredi- 

 tary relationship throughout the animal series — the origin of the 

 diverse forms by gradual change. (Figs. 172, 174.) 



D. Problems of Development 



Embryology is something more than the description of the ka- 

 leidoscopic series of stages which seem to melt one into the other 

 as development progresses. It attempts, especially at the present 

 time, to look below and beyond structure to the processes in- 

 volved, and to determine how the sequence of events is brought 

 about. This is but a repetition of the stages of progress in all 

 science; a passage from the descriptive to the experimental. The 

 results thus far secured have raised many, and answered some prob- 

 lems of development of great practical importance and theoretical 

 interest. The outline of one broad problem may serve as an 

 example. 



From what the pioneer students of embryology during the 

 seventeenth and eighteenth centuries saw, or thought they saw, 

 with simple lens and newly invented compound microscope, there 

 were gradually formulated two opposing views of development 



