CHAPTER VI 

 THE MULTICELLULAR ANIMAL 



Over the structure of the cell rises the structure of plants and ani- 

 mals, which exhibit the yet more complicated, elaborate combinations 

 of millions and billions of cells coordinated and differentiated in the 

 most extremely different ways. — Herlwig. 



It has been pointed out that all organisms consist either of one 

 free-living cell or of many cells, and some idea has been gained of 

 unicellular animals from our survey of the Protozoa, so we are now 

 in a position to consider the origin and organization of the individual 

 in the Metazoa, as the multicellular animals are sometimes called. 



Every Metazoan individual, with exceptions to be noted later, 

 begins its existence as a single cell which has been set free as such 

 from the parent, or which has been formed by the fusion of two 

 cells, or gametes, each typically derived from a separate parent 

 individual: one male, the other female. The former method is 

 known as uniparental, or asexual, reproduction and the latter 

 as biparental, or sexual, reproduction. The union of male and 

 female gametes (sperm and egg) in sexual reproduction to form 

 the zygote is termed fertilization. Roth asexual and sexual 

 methods are common in many of the lower animals, but in higher 

 forms, including Man, only sexual reproduction prevails, so em- 

 phasis at present will be placed on the latter. (Fig. 133.) 



The most remarkable fact about the zygote (fertilized egg) is its 

 power to develop into an organism similar to the parents from 

 which its components, the gametes, have separated. The zygote is 

 set, one may say, to go through a series of changes which trans- 

 form an apparently simple cell into an obviously complex multi- 

 cellular animal with all the characteristics of the species. It is 

 important, at this point, to review the typical method by which 

 the development of the adult is accomplished. 



A. Development 



Briefly, the general method of animal development is cell divi- 

 sion accompanied by growth and differentiation. The zygote 



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