THE EARLY DEVELOPMENT OF THE FROG 103 



grows downward more rapidly than the remainder, due in part 

 to the inflowing of the lateral portions toward the mid-line. 

 This process is termed concrescence, or confluence, as in Amphi- 

 oxus. In this way the materials found in the lateral, as well 

 as posterior, regions of the germ ring are drawn to the median 

 region, and as the ring there extends backward or downward, 

 a thick median strand of tissue is left, from which are developed 

 later, when the embryo begins to form, the rudiments of many 

 of the important axial organs. The downward progress of the 

 lateral and ventral margins of the blastopore is quite slow 

 comparatively, so that the closure seems to occur mainly to- 

 ward the lower pole of the gastrula. That is, as the diameter 

 of the circular blastopore diminishes, its center, which is the 

 center of the yolk plug, moves toward the lower pole and may 

 finally reach this or even pass beyond it a short distance up on 

 the opposite side. The form of the blastopore changes mark- 

 edly during the later phases of its closure. The final rapid 

 approach of the lateral margins alters its outline from a circle, 

 so that it becomes ovoid and finally quite elongated and slit- 

 like, in the direction of the sagittal plane of the gastrula 

 (Figs. 35, 38, A). But before this occurs the blastopore is 

 carried back near its point of origin by the rotation of the whole 

 gastrula to be described shortly. 



Sections through the gastrula during this period of closure of 

 the blastopore show that many important processes are going 

 on internally. Continuing from the stage described, where the 

 archenteron had become a curved crevasse putting the invagi- 

 nated and delaminated endoderm of its outer wall into connec- 

 tion, we find that the essential process of gastrulation is con- 

 tinued chiefly by the rising of the yolk cells from the floor of the 

 blastocoel in advance of the archenteron, so that the inner 

 limit of this cavity is carried upward under the animal pole 

 and then beyond, toward the side of the gastrula opposite that 

 of the first appearance of the archenteron (Fig. 32, C-F). The 

 yolk cells at the same time are carried around the front of the 

 advancing archenteron and form the outer wall of the primary 

 gut cavity. This rearrangement of the yolk cells, involving 



