CHAPTER XX 

 DEVELOPMENT 



The student of Nature wonders the more and is astonished the less, 

 the more conversant he becomes with her operations; but of all the 

 perennial miracles she offers to his inspection, perhaps the most 

 worthy of admiration is the development of a plant or animal from its 

 embryo. — Huxley. 



The new individual, established by the orderly merging of a cell 

 detached from each parent in sexually reproducing species, has 

 before it first of all the problem of assuming the adult form by a 

 complicated developmental process. As we have seen, this in- 

 volves cleavage of the egg, followed, in the Metazoa, by blastula 

 and gastrula stages during which the primary germ layers are 

 established — the fundament out of which the definitive form, 

 organs, and organ systems of the adult are evolved. The descrip- 

 tion and comparison of these processes in different organisms con- 

 stitute the content of one aspect of embryology. (Fig. 288.) 



It is unnecessary — indeed, it is impossible — for us to survey 

 the immense field included under embryology. We must be satis- 

 fied with the realization that animal development, though it varies 

 widely in producing the immensely diverse body forms, exhibits 

 throughout a thread of similarity in its fundamental features; and 

 the appreciation of the marvellous intricacy of the developmental 

 process even in the lowly animals. This perhaps may be gained 

 by concrete examples — first the embryological development of 

 the Earthworm from the zygote to the establishment of the gen- 

 eral body plan. 



A. Embryology of the Earthworm 



The egg of the Earthworm, after fertilization, proceeds to divide 

 first into two cells, then four cells, eight cells, and so on, with more 

 or less regularity, until a condition is attained in which many rela- 

 tively small cells are arranged about a central cavity. This stage 

 of the embryo will be recognized as the blastula. 



The various cells of the blastula appear essentially the same ex- 

 cept that those at one end are somewhat larger than at the other. 



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