Chap. 19 DEVELOPMENT 



Animal hemisphere 

 Gray crescent 



369 



Vegetal 

 hemisphere' 



Stage 2. 1 hr post- 

 fertilization. Right 

 side view. 



Stage 3. First cleavage Stage 4. Second Stage 5 Third 

 at 3.5 hrs. Posterior cleavage at 4.5 hrs. cleavage at 5.4 

 view. Right side view. hrs. 8 cells. 



Stoge 7. Fifth cleavage. Stoge 10. Earliest Stage II. Extension Stage 12. Complete 

 32 cells at 7 hrs. involution of dorsal of dorsal to lateral lip involution, en- 

 lip at 26 hrs. Pos - lips at 34 hrs. circling yolk at 

 terior view. Posterior view. 42 hrs. 



Fig. 19.7. General survey of the early development of the leopard frog seen in 

 external views. The stages are selected from many intermediate ones. Sioge 2, the 

 fertilized egg. Polar bodies not shown. Stages 3, 4, and 5; cleavage. Continued di- 

 vision creates smaller cells. Where yolk is most abundant, in the vegetal hemi- 

 sphere, division is slower and the cells are larger. Stage 7, early blastula. Stages 10, 

 11, and 12; gastrula. The crescentic groove (10) becomes a ring (12) as the mi- 

 nute rapidly dividing cells of the ectoderm grow over and around the more slowly 

 dividing yolk-filled cells of the endoderm. These and the cells which will form 

 notochord and mesoderm are thus turned inside (involution). In 72, only the yolk 

 plug, a pinhead of endoderm, is visible. The opening decreases but remains for a 

 time as the blastopore. (Courtesy, Rugh: The Frog. Philadelphia, The Blakiston 

 Co., 1951.) 



the processes by which the single layered blastula is converted into the animal 

 with a definite ectoderm and endoderm and chordamesoderm about a future 

 digestive cavity. In the embryos of hydra, starfish, amphioxus and others there 

 is little yolk in the vegetal region of the embryo to hinder the ingrowth of cells 

 that creates the pioneer food cavity. However, in the frog the cells in the 

 vegetal area are burdened by yolk and do not grow inward so readily. Actu- 

 ally, the embryo frog has to swallow a lump of yolky food at its rear end. This 

 process begins with the ingrowth of cells that results in the appearance of the 

 crescentic groove at the junction of the animal and vegetal hemispheres (Fig. 

 19.7). The crescentic groove deepens because the cells multiply so fast that 

 they not only turn inward, but grow farther and farther over the yolk-filled 

 cells which are also turning in. While this is going on, the horns of the crescent 

 grow toward one another and finally complete a circle. At the same time, the 

 rim continues to close in, and makes the circle smaller and smaller. By now 

 less than a pinhead of white cells, the yolk plug, is visible, and presently not 

 even this because the dark rim has closed the blastopore. The food-filled cells 



