REPRODUCTION AND DEVELOPMENT IN CHORDATES 



we see externally is the constriction of the cytosome during the telophase of 

 mitosis; the orderly separation of half-chromosomes occurs before this. Dur- 

 ing the early divisions all the cells divide at so nearly the same rate that it 

 appears as if the zygote were being cut with a knife into smaller and smaller 

 parts. If the zygote is visualized as a globe with the north pole of the 

 earth representing the animal pole of the zygote and the south pole repre- 

 senting the vegetal pole, it may be easier to understand how the zygote is 

 divided during cleavage. 



^ Polar body 



, Perivitelline 

 space 



Polar bodj' 



Ectoderm Gastrocoel Endoderm 



Dorsal 

 Germ ring lip 



Germ ring 



Blastopore Neural plate 



Ectoderm 



Anterior end 

 Endoderm 



Gastrocoel 

 Ventral surface 



Chordamesoderm 



Blastopore 



H 



Fig. 5.14. Early development in the amphioxus. A, one-cell stage, in section; the arrow 



indicates the egg axis. 5, eight-cell stage, from the surface. C, 32-ceU stage, from the 



surface. Z), early blastula, in section. E, late blastula, as if cut in half. F, early gas- 

 trula, in section. 6', gastrula, from the surface. //, late gastrula in section, oriented to 



show dorsoventral and anteroposterior axes. (.4, B, and G, from E. G. Conklin, 1933, 



Journal oj Expenmenlal ^oology, vol. 64; C-F and H from E. G. Conklin, 1932, Journal 

 of Morphology, vol. 54.) 



147 



