Diagram of Early Stages

Developmental stages of amphibians.

(Based on Rugh, 1948)

(A) Stage 1: Unfertilized egg. (B) Stage 2: Fertilized egg. There is a shifting of cytoplasm that uncovers a lighter area of pigmentation called the gray crescent. This is the precursor to the notochordal mesoderm. (Does not appear in the African clawed toad, Xenopus laevis. (C) Stage 3: 2 Cells. The first cleavage plane usually bisects the gray crescent.
(D) Stage 4: 4 Cells. The second cleavage plane is parallel to the first. (E) Stage 5: 8 Cells. The third cleavage plane is perpendicular to the first two. (F) Stage 6: 16 Cells. The fourth cleavage plane is parallel to the first two. (G) Stage 7: 32 Cells. The fifth cleavage planes are parallel to the third. (H) Stage 8: Mid-cleavage. Cleavage is faster in the animal hemisphere than in the vegetal hemisphere, so the cells in the animal half are smaller and more numerous than those in the vegetal half. (I) Stage 10: Dorsal lip forms. Gastrulation is just beginning as cells involute forming the dorsal lip of the blastopore. (J) Stage 11: Mid-gastrula. The embryo is shown from the lower side. Cells are involuting from all sides of the blastopore. Large, yolky endoderm cells caught in the middle form the yolk plug. (K) Stage 12: Late gastrula. The embryo is shown from the lower side. As gastrulation proceeds, the yolk plug gets smaller. (L) Stage 13: Neural plate forms. The embryo is shown from the dorsal side. Neural ectoderm begins the process of forming a neural tube. (M) Stage 14: Neural folds form. The embryo is shown from the dorsal side. The edges of the neural plate are folding upwards. (N) Stage 15: Rotation begins. The embryo shown from the dorsal side. The neural folds are closing to form a neural tube. The epidermis of the embryo is ciliated, and the beating of the cilia cause the embryo to rotate within its vitelline envelope. (O) Stage 16: Neural tube. The embryo is shown from the dorsal side. The neural tube is now closed. The bulge of the forming eyes can be seen at the anterior end.