TJd) a 'Embryonic Development; 

 Differentiation of the Tissues 



AT CONCEPTION, each animal, whether 

 starfish, frog, or man, begins life as a single 

 cell. The fertilized egg, or zygote, soon 

 divides, however, and it continues to divide, 

 forming the many cells of the developing 

 embryo. The cells become arranged and dif- 

 ferentiated according to a precise and intri- 

 cate pattern in each particular species; and 

 a study of these beautifully integrated pro- 

 cesses constitutes a fascinating area in biol- 

 ogy. The events of embryonic development 

 sometimes can be observed under the micro- 

 scope, because the embryos of many marine 

 animals are small and transparent. A fertil- 

 ized egg placed in a drop of sea water on a 

 microscopic slide continues to cleave. One 

 can watch as the cells become regimented 

 and differentiated in the various developing 

 organs and tissues. 



First to be considered here are the morpho- 

 logical aspects of development, which con- 

 stitute descriptive embryology. In Chapter 

 27, an analysis of some of the problems of 

 experimental embryology will be under- 

 taken. 



Each parent contributes just one haploid 

 cell, or gamete, to the constitution of the 

 zygote and embryo. Also, in a vast majority 

 of cases, the female gamete is quite different 

 from the male. Each has specialized functions 

 and each has developed a specialized form 

 (p. 59). 



The Egg Cell, or Ovum. Typically the egg 

 is a large, nonmotile, haploid cell. The size, 

 however, varies greatly depending upon how 

 much yolk material is present. Eggs may be 

 classified, in fact, on the basis of amount and 

 distribution of this material (Fig. 15-1). 

 Essentially the yolk represents a stored re- 

 serve of organic matter. This is utilized dur- 

 ing the very active processes of embryonic 

 development. 



The haploidy, of course, results from the 

 formation of the polar bodies (Figs. 15-2 and 

 26-3). These meiotic divisions provide for 

 the selection of a full haploid set of chromo- 

 somes for transmission to the incipient new 

 generation, without sacrificing much of the 

 organic reserves that have been built up by 

 the maturing oocyte (p. 53). At the same 



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