EARLY DEVELOPMENT IN THE FROG. 



345 



the data on which are fully as striking and significant as those 

 on differential inhibition. But their consideration is reserved 

 for future consideration along with a detailed discussion of 

 differential susceptibility in the later stages of development in 



FIG. 24. Cyclopic frog embryo. (Experiment R 122.1.) 



the frog's egg. However, for the sake of completeness a few of 

 the more striking abnormalities of later development may be 

 mentioned. These are: microcephalic forms with such bilateral 

 structures as optic vesicles, ventral suckers, and nasal pits in all 

 degrees of approximation from nearly normal to complete "fu- 

 sion." 1 These forms are seen somewhat more frequently in 



FIG. 25. Spina bifida, microcephalic, ventral suckers nearly fused. Twelve 

 hours' exposure (from early gastrula) to M/3OO NaOH; four days in water. (Ex- 

 periment R 125.4.) 



m/io LiCl than in other agents but they have been obtained in 

 varying percentages in all of the agents used to modify develop- 

 ment. Several of the forms are illustrated in Figs. 22-25. The 

 history and treatment of the eggs is given in the legends to the 

 figures. 



In addition to the marked apical inhibitions just mentioned 

 the later stages show also considerable median dorsal inhibition 



1 The term "fusion" is used in a purely descriptive sense for the eye of cyclopic 

 forms, single median nasal pits, or single median ventral suckers. Whether the 

 primordia of the organs mentioned arise as paired structures and later fuse to form 

 a single median structure, or whether a single primordium arises that later divides 

 into two parts in normal individuals, but which do not separate under strongly 

 inhibiting conditions, is a question for future consideration. 



