THE TOPOGENESIS OF THE EMBRYO 



97 



Fig. 34. Diagram of gastrulation in amphibians, (a) blastula, halved 

 in the median plane, with thin roof and thick floor (coarse hatching: 

 animal field; fine hatching: marginal zone, the outline of cells is 

 indicated in the vegetative material), blastopore furrow below at 

 right; (a-e) invagination of the archenteron, starting from the 

 blastopore furrow; inward movements of the vegetative material; 

 rolling in of the marginal zone material over the edge of the blasto- 

 pore; (/) cross section through the same stage as in (e), the trough 

 shaped endoderm forms the floor and the sides of the archenteron, 

 the prospective notochord and mesoderm its roof, whereas the 

 ectoderm covers the outer surface of the germ; (g) dorsal view of 

 neurula, showing the neural plate, surrounded by a raised fold 

 which separates it from the rest of the ectoderm; (h) cross section 

 through the same stage as in (g); the endoderm has closed dorsally, 

 forming the gut, the archenteron roof has differentiated into noto- 

 chord and mesoderm, the ectoderm into neural plate and skin ecto- 

 derm. After Hamburger and Mayer. 



Raven - Outline Physiologie 



