THE EARLY DEVELOPMENT OF THE FROG 95 



of cells derived from the animal region of the egg; these vary 

 considerably in size and form, are irregularly and loosely ar- 

 ranged, and are roughly disposed in two sheets, one lining the 

 blastocoel and one covering the surface. The thicker floor 

 of the blastocoel is formed of the larger and less numerous 

 vegetative cells. As the number of cells continues to increase 

 several processes go on together. The small cells of the animal 

 region divide the more rapidly and as they multiply they move 

 gradually from the pole toward the equator, causing a thinning 

 of the roof and a thickening of the walls of the blastoccel. 

 This thickening toward the equator of the blastula is aug- 

 mented by the rapid multiplication of cells there so that sec- 

 tions soon show a thicker ring, not very definitely delimited, 

 of actively dividing cells forming what we may call the germ 

 ring or growth zone, such as that described in the blastula and 

 gastrula of Amphioxus (Fig. 31, C). This thinning of the 

 animal pole increases the eccentricity of the blastocoel, which 

 has meanwhile increased considerably in size. The later 

 blastula increases somewhat in diameter, and accompanying 

 this is the absorption or infiltration of water into the blastoccel, 

 a part of the fluid content of which is, however, the secretion 

 of its walls. 



The germ ring is obviously formed of material from the 

 animal pole of the egg, and apparently the substance con- 

 tained in it can be distinguished at least as early as the eight 

 cell stage, where it forms the upper quartet (micromeres) and 

 the upper parts only of the lower quartet (macromeres) (Fig. 

 34). As the germ ring approaches the equator, one side (that 

 of the gray crescent) commences to extend downward faster 

 than the remainder; subsequent development proves this to 

 be the posterior side. Soon the entire germ ring passes the 

 equator, and by the time the blastula period is ended, it 

 reaches, on the posterior side, a point about half way between 

 the equator and the lower pole. These later phases in the 

 downward movement of the animal cells can be observed 

 externally, for these cells are easily distinguishable from the 

 true lower pole cells on account of their dense pigmentation. 



