EMBRYOLOGY AND EVOLUTION 



The Course of Development 



Let us consider in brief outline some of the main facts 

 of individual development. In practically all animals and 

 plants development begins widi the fertilization of an tgg. 

 William Harvey (1651), the discoverer of the circulation 

 of the blood, expressed this fact in his famous dictum, 

 "Omne vivum ex ovo." The tgg is a cell with the structures 

 and functions that are characteristic of cells in general ; that 

 is, it contains protoplasm, "the physical basis of life," and 

 this protoplasm is differentiated into a nucleus and a cell- 

 body, and each of these contains other smaller units, many 

 of which differ one from another. But these units are not 

 adult parts in miniature, as the ' preformationists" supposed; 

 they are merely the elements out of which adult parts are 

 built; they are like the letters of the alphabet out of which 

 are built words and books and literatures. 



In all animals the process of fertilization is practically the 

 same, consisting essentially in the union of the nuclei of tgg 

 and spermatozoon. In almost all multicellular animals the 

 tgg at or near the time of fertilization gives off two minute 

 cells, the polar bodies, which are rudimentary eggs and take 

 no part in development. The fertilized tgg undergoes re- 

 peated divisions or cleavages, forming a mass of cells, usually 

 a hollow sphere, which is called a hlastula, and this in turn 

 becomes a gastrula by the formation in It of a gastric cavity. 

 So far all animals, from sponges to men, travel the same road, 

 although in every group there are minor peculiarities; they 

 travel the same road, but they do not all follow in exactly 

 the same tracks. The germ cells, the cleavage, the blastula, 

 and the gastrula show characteristic differences in different 

 phyla or classes, but their resemblances are more significant 

 than their differences, and within the same phylum these 



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