CHAPTER II 



CLEAVAGE OF THE FERTILIZED OVUM AND ORIGIN OF THE 



GERM LAYERS 



CLEAVAGE 



The processes of cleavage, or 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. Cleavage may be 

 modified, however, by the presence in the ovum of large quantities of nutritive 

 yolk. In many vertebrate ova the yolk collects at one end, termed the vegetal 

 pole, in contrast to the more purely protoplasmic animal pole. Such ova are said 

 to be telolecithal. Examples are the ova of fishes, amphibians, reptiles, and birds. 

 When very little yolk is present, the ovum is said to be isolecithal. Examples 

 are the ova of Amphioxus, the higher mammals, and man. The typical processes 

 of cleavage may be studied most easily in the fertilized ova of invertebrates 

 (Echinoderms, AnneHds, and Mollusks). Among Chordates, the early processes 

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

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

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

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

 ovum of the bird and of the lower mammals, and the influence of this condition 

 persists. 



Cleavage in i^oiphioxus. — The ovum is essentially isolecithal since it contains 

 but little yolk (lig. 15). Abou| one hour after fertilization it divides vertically 

 into two nearly equal daughter cells, or blastomeres. The process is known as 

 cell cleavage, or segmentation, and takes place by mitosis. Within the next hour 

 the daughter cells again cleave in the vertical plane, at right angles to the first 

 division, thus forming four cells. Fifteen minutes later a third division takes 

 place in a horizontal plane. As the yolk is somewhat more abundant at the vege- 

 tal pole of the four cells cne mitotic spindles lie nearer the^animal pole. Conse-, 

 quently in the eight-celled stage the upper tier of four cells is smaller than the 

 lower four. By successive cleavages, first in the vertical, th. _i in the horizontal 

 plane a 16- and 32-celled embryo is formed. The upper two tiers are now smaller, 



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