M 



m.s 



FIG. 152. Early development of Amphioxus. 



A, the oosperm ; B, stage with two blastomeres ; C, stage with four blastomeres ; 

 D, stage with eight blastomeres ; E, stage with sixteen blastomeres ; F, stage 

 with thirty-two blastomeres. cut in half vertically ; G, blastula stage, cut in half ; 

 H, early stage of gastrulation, cut in half ; /, young gastrula in longitudinal 

 section ; /, older gastrula in longitudinal section ; K, L, M, transverse sections 

 of older embryos, showing the formation of the coelomic pouches or enterocceles, 

 notochord, and medullary tube ; N, longitudinal section of embryo of about the 

 same age as L ; O, side view of embryo of same age as N, with the ectoderm 

 stripped off from one side to show the mesodermic segments formed from the 

 coelomic pouches. 



578 



