CHAPTER II 



SEGMENTATION OF THE FERTILIZED OVUM AND 

 ORIGIN OF THE GERM LAYERS 



SEGMENTATION 



The processes of segmentation, not having been observed in human ova, 

 must be studied in other vertebrates. It is probable that the early development 

 of all vertebrates is, in its essentials, the same. It is modified, however, by the 

 presence in the ovum of large quantities of nutritive yolk. In many vertebrate 

 ova the yolk collects at one end, the vegetal pole. Such ova are said to be 

 telolecithal. Examples are the ova of Amphioxus, the frog and bird. When 

 very little yolk is present, the ovum is said to be alecithal (no yolk). Examples 

 are the ova' of the higher mammals and man- The typical processes of cleavage 

 may be studied most easily in the fertilized ova of invertebrates (Echinoderms, 

 Annelids, and Mollusks). Among Chordates, the early processes in develop- 

 ment are primitive in a fish-like form Amphioxus. The yolk modifies the 

 development of the amphibian and bird's egg, while the early structure of the 

 mammalian embryo can be explained only by assuming that the ova of the 

 higher Mammalia at one time contained a considerable amount of yolk like the 

 ovum of the bird and of the lower mammals. 



Amphioxus. — The ovum is telolecithal, but contains little yolk (Fig. 14). 

 About one hour after f ertilization it divides vertically into two nearly equal daugh- 

 ter cells. The process is known as cell cleavage, or segmentation and takes place 

 by mitosis. Within the same interval of time the daughter cells cleave in the 

 same plane, forming four cells. Fifteen minutes later a third segmentation takes 

 place in a horizontal plane. As the yolk is more abundant at the vegetal poles 

 of the four cells the spindle lies nearer the animal pole. Consequently in the eight- 

 celled stage the upper tier of four cells is smaller than the lower four. By suc- 

 cessive cleavages, first in the vertical, then in the horizontal plane a 16- and 32- 

 celled embryo is formed. The upper two tiers are now smaller and a cavity, the 

 blastocoel, is enclosed by the cells. The embryo is called a morula (mulberry). 

 In subsequent cleavages, as development proceeds, the size of the cells is di- 

 minished while the cavity enlarges (Fig, 14). The embryo is now a blastula, 

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