48 INTRODUCTION TO EVOLUTION 



and sea urchin eggs among invertebrates, or amphioxus eggs in Phylum 

 Chordata. In its essential features, however, the sequence of changes 

 shown characterizes all animals. Fig. 4.2 is related to Fig. 4.1 in the fol- 

 lowing manner. Stage a of Fig. 4.2 represents a fertilized ovum like those 

 shown in the bottom row of Fig. 4.1. Stages / and g represent the stage 

 shown in the second horizontal row of Fig. 4.1. Thus stages b through e 



Animal pole 



Vegetal pole 



Animal pole 



Ectoderm 



Cleavage Vegetal 

 cavity pole 



'Cleavage 

 cavity 



Endodefm y 



Cleavage m 

 cavity lJ 



Archenteron 



FIG. 4.2. Typical early embryonic development; g, h, and i are shown 

 cut in half. (From Guyer, Animal Biology, Harper & Brothers, 1948, 

 p. 476.) 



Stand in between the bottom and the second rows of Fig. 4.1, and stages 

 h and / are slightly later stages than the stage shown in the second row of 

 that figure. 



Fig. 4.2 demonstrates that the fertilized ovum undergoes a series of cell 

 divisions. The original single cell divides into two (b), then each of these 

 two divides into two in turn, the result being a four-celled stage(c). The 

 cells continue to divide, so that we have successively an eight-celled stage 

 (d), a sixteen-celled stage (e), a thirty-two-celled stage, and so on. As 



