MODIFICATION OF DEVELOPMENT IN THE FROG A479 
to attempt any detailed description of the great variety of forms 
belonging to any of the four types. Hence, in the following four 
sections only the characteristic general features of the types will 
be pointed out. The conditions under which the different types 
occur may be seen from the legends to the figures and by reference 
to the protocols of experiments. 
Differential inhibition. The modified types that fall into this 
class, while highly variable, have the following common charac- 
teristics: Physiologically more active regions suffer relatively 
greater retardation than less active regions. Among the more 
conspicuous external features of differential inhibition forms seen 
in postgastrula stages are these: Embryos with an elongated 
persistent yolk plug and retarded apical region (figs. 1, 2), or 
with a very marked retardation of apical dorsal parts and showing 
an equatorial blastopore with a persistent yolk plug (figs. 7, 8, 
9). The later type grades back, in more completely inhibited 
forms, to a condition where apical differentiation does not pro- 
ceed and the embryo dies as an equatorial gastrula. Where de- 
velopment proceeds further from such a condition, the embryo 
appears to elongate in the direction of the old egg axis, resulting 
in types that may appear to be radially symmetrical or may pro- 
ceed to the formation of embryos similar to those shown in figures 
3, 4, 5, 6. In somewhat later stages one characteristically finds 
microcephalic or even anencephalic embryos with such structures 
as optic vesicle, olfactory pits, and ventral suckers, when they 
appear at all, in various degrees of approximation to complete 
‘fusion’ (figs. 10 to 32). In the absence of suitable conditions 
or opportunity for recovery, such embryos are usually dorsally 
concave, with the tail in some cases extending dorsalward at 
an angle of 90° or less with the body (fig. 22). The yolk plug 
may or may not be persistent, but the blastopore always, when 
it closes at all, does so relatively much later than in normal em- 
bryos. Spina bifida forms are of common occurrence (figs. 23, 26). 
These various types are easily referable to the relatively greater 
retardation of physiologically more active regions, such as apical, 
medial, and dorsal regions, and later, the blastopore lips, pri- 
mordia of optic vesicle, olfactory pits, ventral suckers, etc. 
