Fig. 47 



A to M. Various stages of the development of Amphioxus lanceolatus (mostly 

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

 wvum ; 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 blastoccele, opening to the exterior above and below ; G, a later stage, 

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

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

 pole ; K, gastrula, with widely-open blastopore ; Z, 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, blastoccele; t>p, blastopore; ec, epiblast ; en, hypoblast ; 

 tttic, micromeres ; mac, macromeres ; nec t neurenteric canal ; np, neural plate ; 

 pb, polar bodies. 



