56 EMBRYOLOGY 



respiration. We shall consider this growth problem and the reduplication of 

 the elements in cells in a later chapter and proceed now with an analysis of 

 the problem of differentiation. The essential problem involves the way in 

 which the parts of the egg become different as development proceeds. We 

 shall first study the egg to discover the first difference which appears. By 

 cutting the egg into parts we shall find this first difference. Then, knowing 

 where the first difference is located, we shall experiment further to see how 

 it acts to produce further differences. 



Organization in the amphibian egg 



At this point it is convenient to limit the material under discussion to the 

 amphibian egg. In the last chapter we considered fertilization primarily in 

 the sea urchin egg. After fertilization it is easier to study the amphibian egg 

 because of its larger size. One of the first visible changes in an amphibian 

 egg after fertilization is the appearance on one side of the egg of a region 

 just below the equator. This is called the gray crescent. It marks the polarity 

 of the egg just as the pigment band does in the sea urchin egg. The upper 

 pole in Figure 23, then, is the animal pole, A; the lower pole is the vegetal 

 pole, V ; and the gray crescent is a region just below the equator of the 

 spherical egg. The significance of the gray crescent to embryonic develop- 

 ment can be illustrated very clearly by a simple experiment performed after 

 the first cleavage. 



First cleavage in the amphibian egg is variable in its relationship to the 

 gray crescent. The cleavage plane always passes through the poles, but it 

 may cut through any part of the gray crescent or miss it completely. Let us 

 consider two cases. Suppose, for example, that the first cleavage bisects the 

 gray crescent in the fashion illustrated in Figure 23, and thus each of the first 

 two blastomeres contains a half of the gray crescent, Under these conditions 

 if the blastomeres are separated and isolated, each of these blastomeres will 

 develop into an entire embryo. If, on the other hand, this same operation is 

 performed on an egg in which the gray crescent is cut off into only one of 

 the first two blastomeres we find that only the blastomere which contains the 

 gray crescent forms an entire embryo. The blastomere which lacks the gray 

 crescent forms a simple mass of cells with no internal differentiation. Thus, 

 only blastomeres containing some part of the gray crescent will form an 

 embryo. 



