-f&. 





Fig. 47 



tus (mostly 



.4 to Af. Various stages of the development 'of Ainphioxus lanceolatu 



after Hatschek ; //, /, K, and L modified after Morgan). A, the unsegmented 

 ovum ; B, the first two blastomeres resulting from the first cleavage ; C, stage 

 with four equal blastomeres ; D, eight-cell stage consisting of four smaller 

 cells (micromeres) above and four larger cells (macromeres) below; E, sixteen- 

 cell stage ; F, optical section of thirty-two' cell stage showing the segmentation 

 cavity or blastocoele, opening.to the exterior above and below ; G, a later stage, 

 the blastocoele completely closed in ; H, a blastula shortly before invagination ; 

 J, commencement of invagination of the larger granular cells of the lower 

 pole ; K, gastrula, with widely-open blastopore ; L, later gastrula in which 

 the blastopore has been narrowed by the folding together of the walls ; 

 M, embryo with neural plate and commencing neurenteric canal. ach, 

 archenteron ; be, blastocrele ; bp^ blastopore ; ec, epiblast ; en, hypoblast ; 

 mic, micromeres ; mac, macromeres ; nee, neurenteric canal ; np, neural plate ; 

 pb, polar bodies. 



