64 DEVELOPMENT OF THE FROG'S EGG [Cn. VI 



taken by many of these structures in the embryo resemble 

 the folds that can be produced mechanically by pulling out or 

 pushing in a thin elastic plate of rubber. If this interpretation 

 is true, it means that at different periods in the development, 

 regions of more rapid growth appear, now here, now there, and 

 as a mechanical result of the conditions present, such structures 

 as the medullary folds, the eye-outgrowths, etc., are produced. 

 The cells change their shape in response to surrounding con- 

 ditions, i.e. they do not by their individual activity or move- 

 ment change their shape to produce the successive changes of 

 the embryo, but the shape of many cells is changed as the 

 result of growth or increase in mass of certain regions. For 

 instance, a cell becomes conical not through its own initiative, 

 but because the surrounding pressure forces it into a conical 

 shape. 



THE FORMATION OF THE EMBRYO BY CONCRESCENCE 



The period of overgrowth of the blastopore when the so 

 called process of gastrulation is going on has been described in 

 Chapter V. We may now follow the changes that take place 

 in the interior of the egg during that time. 



When the dorsal lip of the blastopore appears, the cells have 

 shown little tendency to arrange themselves into sheets or 

 layers. However, even when the segmentation-cavity is covered 

 by a roof of small cells, the cells of the outer layer have begun 

 to flatten against one another and to form a thin layer of cells 

 over the outer surface of the black hemisphere. In the lower 

 hemisphere the larger white cells do not show such an arrange- 

 ment. In the equatorial region, where the black and white 

 cells meet, a careful examination of sections will show that 

 there exists a more or less denned ring of cells stretching 

 around the embryo, forming a broad zone (Fig. 15, D). The 

 inner cells of this ring contain a good deal of pigment around 

 the nuclei. The yolk-granules of these inner cells are smaller 

 than the yolk-granules in the large white cells of the lower 

 hemisphere, and the cells of the ring seem to contain also a 

 larger amount of clear protoplasm. This inner zone of cells 

 passes, on the one hand, by insensible gradations into the cells 

 of the outer surface of the ring and internally it is continuous 



