CHAPTER II 



CLEAVAGE OF THE FERTILIZED OVUM AND THE ORIGIN OF THE 



GERM LAYERS 



CLEAVAGE 



THE processes of cleavage, or segmentation, not having been ob- 

 served 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 proto- 

 plasmic 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, Annelids, 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 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 lowest 

 mammals, and the influence of this condition persists. 



Cleavage in Amphioxus. The ovum is essentially isolecithal, since 

 it contains but little yolk (Fig. 14). About 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 vegetal pole of the four 

 cells, the mitotic spindles lie nearer the animal pole. Consequently, in 

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

 four. By successive cleavages, first in the vertical, then in the horizontal 

 plane a 16- and 3 2 -celled embryo is formed. The upper two tiers are now 

 smaller, and a cavity, the blastoccele, is enclosed by the cells. The embryo 

 at this stage is sometimes called a morula because of its resemblance to a 



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