EARLY DEVELOPMENT OF THE FROG. 55 



halfway to the yolk pole. This downward migration displaces the yolk cells 

 in the interior upward, producing an elevation in the floor of the blastoccel. 

 As subsequent development proves, the side where the germ ring reaches the 

 lowest point marks the caudal end of the embryo. During the formation 

 and early migration of the germ ring the blastula increases about one-fifth 

 in size but remains spherical. Some water perhaps filters into the blastoccel, 

 although part of its contents is probably products of cell activities. 



Gastrulation. In the frog as in Amphioxus gastrulation comprises the 

 change of a single-layered structure, the blastula, into a double-layered 

 structure, the gastrula. The processes by which this change is effected are 

 more complex in the frog, the visible factor in the complexity being the 

 greater quantity of yolk. The inert yolk stored within an egg is always an 

 influence in development. 



Viewing first the exterior of the blastula, a slight groove appears on the 

 posterior side across the median sagittal plane at the lowest part of the germ 

 ring, that is, about midway between the equator and the center of the yolk 

 pole. The small pigmented cells bound the groove above, the larger 

 yolk cells below (Fig. 28, H). As development proceeds the groove 

 becomes longer, following the boundary between the two types of cells, 

 which is of course the lower margin of the germ ring. It thus takes on 

 the from of a crescent. Continuing to elongate in the same directon, the 

 two horns of the crescent would eventually meet and the groove would thus 

 become a ring encircling the blastula at the boundary between the pigmented 

 and yolk areas. This actually occurs, but in the meantime the pigmented 

 area extends farther down owing to the descent of the germ ring and the down- 

 ward progress is more rapid on the posterior side where the groove first 

 appeared. The result of this is that by the time the horns of the crescent 

 meet to form a ring, the ring is much smaller than if there had been no down- 

 ward movement; and since the original groove was bounded above by pig- 

 mented cells it now follows that the ring is bounded all round on the outside 

 by pigmented cells. For the same reason the ring is bounded on the inside 

 by yolk cells. These are the only yolk cells now visible on the surface. 

 Subsequently the ring becomes still smaller and then flattened from side to 

 side and finally reduced to a small slit. (See Fig. 30.) 



The changes on the surface are merely partial expressions of the com- 

 plicated processes in the interior. In a sagittal section of the blastula 

 at the time the superficial groove appears, the initial step in these 

 processes can be observed. The groove appears as a slight indentation above 

 which are the smaller cells of the germ ring and below, the larger yolk cells. 

 At this side is seen also the elevation of the floor of the blastoccel caused by 

 the rising of the yolk cells; and there is a slight separation of this elevation 



