268 OUTLINES OF EVOLUTIONARY BIOLOGY 



The possession of food-yolk is, of course, of immense import- 

 ance to the embryo, for the stage which it is able to reach in its 

 development before it has to begin to provide for itself will 

 depend upon the amount of nutrient material which it has at its 

 disposal. The presence of this food-yolk, however, must also 

 act as a mechanical hindrance to the development, just as the 

 supply of provisions which he carries with him must impede the 

 progress of a traveller, though enabling him ultimately to 

 accomplish a longer journey. 



In Amphioxus the egg contains hardly any food-yolk and the 

 process of segmentation and the formation of the blastula and 

 gastrula take place in an almost perfectly typical manner. 

 There is here practically no hindrance to development and no 

 obliteration of the ontogenetic record. There is, however, just 

 sufficient nutrient material to cause some of the cells those 

 which will invaginate to form the hypoblast of the gastrula to 

 be slightly larger than the remainder, which are destined to form 

 the epiblast (Fig. 18, VII, VIII). 



Let us compare with this the corresponding stages in the 

 development of the frog (Fig. 119). Here the egg is, as we have 

 already indicated, much larger, owing to the greater quantity of 

 food-yolk which it contains. This is chiefly collected in the 

 lower half of the ovum, while the upper half is deeply pigmented 

 and thus readily distinguished. Segmentation begins as in 

 Amphioxus by the formation of two vertical clefts, or cell-divisions, 

 one after the other and at right angles to one another, whereby 

 the four-celled stage is reached (Fig. 119, I III). Each cleft 

 begins at the pigmented pole and throughout the whole of the 

 segmentation the yolk-laden half of the egg lags behind the 

 pigmented half because of the mechanical hindrance which the 

 food-yolk opposes to the process of cell-division. The first 

 horizontal cleft (Fig. 119, IV) divides each of the four blasto- 

 meres into two daughter cells of very unequal size, an upper 

 pigmented cell with little food-yolk and a much larger, lower, 

 yolk-laden cell with little pigment. After this, segmentation 

 continues to progress more rapidly in the upper than in the 

 lower half of the egg (Fig. 119, V), until we reach the blastula 

 stage (Fig. 119, VI), where we find the large yolk-laden cells 

 forming the lower half of the wall of the hollow sphere and pro- 

 jecting into and partly obliterating its cavity, while the small 

 pigmented cells form the upper half. 



