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The doctrine of progressive division of labour to which we are come,familiar to all readers. Further, the analogy between the economic divisionlabour and the "physiological division of labor" so striking that for a long time alreadyattracted the attention of naturalists and introduced into science the expression of it.Therefore, there is no need to indulge in unnecessary details whenthe study of this part of our subject. We will udovolstviem showingseveral common and significant facts are not conspicuous whena superficial consideration of the case. Throughout the animal Kingdom, starting with the Coelenterata, the first phase of developmentone and the same. In the Bud of a polyp, as in the human egg massof cells, of which has formed a living creature, ulagamperipheral layer of cells, slightly razlicitih from other cells,this layer is surrounded by, subsequently it is divided into two layers:internal, being in touch with the yolk and are called mucouslayer, and an outer, exposed to surrounding influences and called serouslayer - or, in the words of Professor Huxley, used them whenthe description of the process of the development of Hydrozoa, formed endoderma and ectoderma. Itprimitive separation indicates the main difference of the parts in the futurethe body From the mucous layer, or endoderm's, develops nutritionalthe apparatus, while the serous layer, or ectoderm's developing apparatusoutside activities. From the first formed the bodies, whichthe food is prepared and absorbed, oxygen is drawn and the blood is purified,while the latter is formed of the nervous, muscular and skeletal systemsthe United action which are movements of the body as a whole. Thoughthe definition is not strictly correct, as some authorities have inits composition both main shell, but the high authorityagree that it can be taken as a General distinction.
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