THE FROG, A TYPICAL VERTEBRATE 



265 



150) the nuclear wall breaks down, a spindle-shaped structure of 

 threads with a starlike aster at each end is formed, and the prin- 

 cipal nuclear substance, called chromatin, forms a certain num- 

 ber of rodlike bodies, the chromosomes (Fig. 150, A-D). The 

 chromosomes split and one half of each is drawn to either end 



ect 



Idm 



FIG. 152. Development of the embryo of the frog. 



A. Section of blastula. bl.coel, blastocoel; mi, micromeres; mg, macro- 

 meres. 



B. Formation of medullary groove, md.gr, and medullary fold, md.f ; yk.pl, 

 yolk-plug. 



C. Section of egg in stage B to show germ-layers, bl.coel, blastocoel; blp, 

 blastopore ; ect, ectoderm ; end, entoderm ; ent, enteron ; mes, mesoderm ; 

 nch, notochord ; yk.pl, yolk-plug. 



D. Older embryo, br.cl, branchial arches; stdm, stomodaeum; t, tail. 



E. Newly hatched tadpole, br.l, br.2, gills; e, eye; pcdm, proctodaeum ; 

 sk, sucker; stdm, stomodaeum; t, tail. (From Parker and Haswell.) 



of the spindle (E, F, G). Each group of chromosomes then be- 

 comes the center of a new nucleus (H, I) . In this way the single 

 nucleus forms two. 



Externally a constriction appears around the diameter of the 

 egg and the egg is pinched into two equal parts which remain 



