54 THE FROG 



observe a small excentric cavity, the gastrocoele. 

 It is lined with minute white cells, the endoderm. 

 The third germ layer, the mesoderm, may be shown 

 in demonstration by prepared sections. 



Drawing 31, Draw these stages including the hemi- 

 sections (9) in such a way as to show the greatest num- 

 ber of structures on each. Make each drawing about 

 two inches in diameter, 



2. Early Embryos and Larvae 



Examine the following stages: 



337. An early embryo (neural groove stage). Note on 

 the dorsal aspect the neural folds, meeting at the 

 cephalic end, and approximating each other at the 

 caudal end. Between the neural folds is the broad 

 neural plate. The blastopore is a narrow slit be- 

 tween the caudal ends of the neural folds. What has 

 become of the yolk-plug? 



338. An early embryo (neural tube stage, 3 mm. long). 

 The neural folds have now united to form a neural 

 tube. Note in the head region, the stomodaeum (the 

 future mouth) , a narrow slit on the ventral surface, 

 enclosed by ridges which will become the jaws. The 

 beginnings (primordia) of the oral suckers may be 

 seen ventral to the stomodaeum. On each side of 

 the head is a bulge caused by the developing eye. 

 Behind this is a swelling known as the gill plate, 

 with five furrows, the visceral (gill) grooves. Be- 

 hind the last is a swelling caused by the pronephros. 

 On the dorsal side of the body are slight furrows 

 indicating the presence of somites. A ventral bulge 

 is the yolk-sac. There is a slight tail bud, beneath 

 which is a small vent. 



