GENERAL ZOOLOGY 



dividual will be discussed later (p. 159), but it mav be noted here that un- 

 due evaporation is prevented during the development of terrestrial vertebrates 

 by egg membranes and shells or by the retention of the embryo in a uterus. 

 The range of temperature is determined in fishes, amphibians, and reptiles by 

 the season during which the eggs are laid, and unseasonal temperatures may 

 kill such developing eggs. Birds, of course, incubate their developing eggs; 

 in mammals, the mother's body regulates the temperature of the embryos in 

 the uterus. In our discussion of. development we shall assume that all en- 

 vironmental conditions are normal. The development of a single cell into a 

 complex, highly differentiated animal is one of nature's marvelous pageants, 

 a series of events that occurs in such an orderly fashion as to fill the observer 

 with awe. 



The pattern of earlv development is correlated very closely with the amount 

 of nutritive material stored in the oocyte during its growth period in the 

 ovarv. In the Chordata there is a wide range in the quantity and position of 

 the yolk in eggs. 'I'he amphioxus (p. 550), for example, has an isolecithal 

 egg, so called because the relatively small amount of reserve nutrients is dis- 

 tributed almost uniformly throughout the cytosome. Among the vertebrates 

 the frog has a telolecithal egg in which a considerable quantity of volk is 

 stored more abundanlK in one half, the so-called vegetal hemisphere, than in 

 the other half, or animal hemisphere, where the nucleus is always found 

 (F'ig. 5.3/?). A more pronounced polarization of nucleus and yolk is to be 

 seen in the telolecithal eggs of fishes, reptiles, and birds (Fig. SAB and C). 

 In such eggs the nucleus is located near the animal pole surrounded by a 

 relatively small amount of cytoplasm forming the blastodisk, and the large 

 cvtosome is packed with yolk enclosed by a very thin layer of cytoplasm, 

 which lies at the surface of the- egg. The egg-laying mammals produce 

 telolecithal eggs like those of reptiles; the placental mammals have isolecithal 

 eggs in which reserve nutrients are stored in varying amounts but never in 

 large quantity. 



As the sequence of events during the development of chordates is described, 

 comparisons will be made between certain representative forms to call atten- 

 tion to the fundamental similarity of development in all, as well as to indicate 

 special differences. Development is a continuous process, although it can be 

 divided into stages for purposes of discussion. In the account which follows, 

 the early stages characterized only by cell division are de.scriljed under that 

 heading. The stages during which the conspicuous mass movements of cells 

 occur are described under the heading of cell localization; the.se movements 

 separate the so-called germ layers and establish the body plan, as well as the 

 primordia of all the organ svstems. The final stages in which cytosomal 

 differences appear in cells, with the production of tissues, are summarized 

 under the heading of cell differentiation. 



Cell Division: Cleavage. Cleavage follows activation and consists of a 

 series of cell divisions. Cell division, of course, occurs during other periods, 

 but during cleavage it is the only visible indication of development. What 



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