Christian faith in an age of science . ns, have completely lost the wings. Thepresence of a rudimentary organ marks an interme-diate stage between the complete and functional devel-opment of the organ and its total loss. Other indications of genetic relationship betweendifferent species are furnished by the facts of embry-ology. All animals above the unicellular protozoacommence life in the condition of a single cell, theovum, which is obviously a condition essentially similarto the permanent condition of the protozoa. Some-what later, in the development of the multicellular ani-mals, appears
Christian faith in an age of science . ns, have completely lost the wings. Thepresence of a rudimentary organ marks an interme-diate stage between the complete and functional devel-opment of the organ and its total loss. Other indications of genetic relationship betweendifferent species are furnished by the facts of embry-ology. All animals above the unicellular protozoacommence life in the condition of a single cell, theovum, which is obviously a condition essentially similarto the permanent condition of the protozoa. Some-what later, in the development of the multicellular ani-mals, appears what has been called the gastrula stage,which appears to be, though with much variation in i8i The Origin of Species detail, essentially the same thing in all. The gastrula,when most typically developed, is a sac formed of twolayers of cells, the outer and the inner layer beingmore or less distinctly differentiated from each is a very noteworthy fact that these tw^o layers ofcells which form the gastrula have respectively the. Fk;. 10.—Six stages in the development of the gastrula in Am-phioxus (a very low type of vertebrate). From Gegenbaurs Vergleiehende Anatomie der Wirbelthiere. same destination in all animals. The outer layeralways forms the epidermis, and, in those animalswhich have a well differentiated nervous system, itforms also the nervous centers and the essential partsof the sense organs. While the tissues derived fromthe outer layer of the gastrula are uniformly the oneswhich are in relation to the external world, the internallayer of the gastrula develops with equal constancy the 182 The Gastrula epithelial lining of the alimentary canal and its append-ages. In some of the lower multicellular animals, as,for instance, the coelentera, the adult developmentpasses little beyond the gastrula stage. The adult bodyconsists of a double-walled sac, whose single cavity isessentially a digestive cavity, and whose wall exhibitsbut slight development of any tissue
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