THE EARLY DEVELOPMENT OF THE FROG 97 



pacted and show no definite arrangement, except that toward 

 the lower pole they gradually increase in size. Although some 

 yolk is contained in all the cells, the larger lower pole cells 

 are particularly rich in deutoplasm and are known as the yolk 

 cells. Immediately below the germ ring the cells are inter- 

 mediate in character and form what is known as the transitional 

 zone. The position of the polar axis in the blastula, with 

 respect to gravity, remains the same as in the egg or the cleav- 

 age period. 



The pigment, while chiefly superficial and in the cells de- 

 rived from the animal pole, externally extends further toward 

 the lower pole than in the egg; it is also found to a limited 

 extent among the animal cells below the surface, and even in 

 the smaller vegetal cells lining the blastoccel, which resemble 

 closely the proper cells of the animal pole. This internal 

 pigment is not derived from that more superficially located 

 in the earlier stages, but it is deposited in situ as a by- or end- 

 product of metabolism. Pigment granules are laid down 

 wherever developmental processes, including cell division, are 

 in rapid progress. And since the smaller cells represent re- 

 gions where cell multiplication has been more active, such 

 cells contain relatively more pigment. This relation between 

 metabolic activity and the accumulation of pigment may 

 explain the pigmentation of the animal pole of the egg itself, 

 although it is customary to refer this to the adaptational 

 relation mentioned previously, a relation which need not be 

 negatived by this method of its formation. The dense pig- 

 mentation of the path of the sperm is also referable to an un- 

 usual degree of metabolic activity. 



The blastula of the frog differs from that of Amphioxus 

 chiefly in that the blastocoel of the former is so decidedly eccen- 

 tric, relatively smaller, and its wall several cells thick, the 

 cells differing greatly in size, and already differentiated into 

 superficial and deeper layers, at least in the animal region. 

 The germ ring (growth zone in Amphioxus) extends completely 

 around the blastula of the frog. These differences for the most 

 part seem to be the direct results of the abundance of yolk in 



