214 THE FROG CHAP, xn 



ventral body walls, and note the coiled intestine, the heart, 

 the internal gills, &c. (Fig. 66, D, D 1 , E.) Sketch. 



It is rather a difficult task, and requires much time, to 

 prepare sections of the early stages of the frog-embryo in 

 order to make out the formation of the ectoderm, endoderm, 

 and mesoderm, the relations of the segmentation-cavity and 

 enteron, and the development of the central nervous system 

 and notochord (Figs. 64, I, K, and 65) ; and directions for 

 the practical study of development are given in Part II, 

 Chapter XIII (p. 615). But if you wish to make the attempt 

 at this stage, proceed as follows : 



Take a few eggs every day from the time they are laid 

 until the tadpoles are hatched : place these on a tile or 

 piece of glass, and with a pair of needles dissect off the 

 gelatinous covering. Then carefully transfer the segment- 

 ing eggs or embryos into corrosive sublimate for a quarter 

 to half an hour, and after washing in running water, put 

 them into weak and then strong alcohol ; stain, imbed, and 

 cut sections as directed on p. 136. 



