266 



PRACTICAL ZOOLOGY 



fastened together. Each of these two cells is provided with one 

 of the two nuclei formed by the division of the single original 

 nucleus. This first cell division is followed by the division of 

 each of the two cells, as shown in the figure (Fig. 151). The 



FIG. 153. Stages in the growth and metamorphosis of the frog tadpole. 

 (From Mivart.) 



cells divide again and again until there are hundreds of cells in 

 the egg. 



These cells become arranged in layers, an outer layer, the 

 ectoderm (Fig. 152, C, ect), an inner layer, the entoderm (ent), and 

 a middle layer, the mesoderm (mes). The cells in each layer 

 differ from those in the others both in their appearance and in 



