DIFFERENTIAL DEVELOPMENTAL MODIFICATION. Ill 259 



stops in early cleavage, a few cleavage furrows may appear about the 

 apical pole, evidently before the agent is fully effective ; but their progress 

 basipetally is soon stopped, and the basal region does not undergo cleavage 

 at all. With certain fat-soluble agents, such as alcohol, this type of modi- 

 fication may perhaps result from a higher concentration in regions of 

 higher yolk content or from a more or less specific action of the fat-soluble 

 agent on this region. 



Under inhibiting conditions permitting continuation of development to 

 the blastula stage or later, the less susceptible yolk cells may proHferate 

 into the blastocoel and in some cases completely obliterate it, the degree 

 of such modification depending on concentration of agent and stage when 

 exposure begins. It has been generally observed that inhibiting agents 

 determine disturbances in gastrulation. According to Bellamy, the dis- 

 tance between apical pole and level of earliest stages of the blastopore 

 decreases with inhibition. The median dorsal lip of the blastopore is most 

 inhibited with the lesser degrees of inhibiting action; and, as gastrulation 

 proceeds, the blastopore takes the form of an inverted U or V and later 

 becomes ovoid in outline with its long axis in the median plane (Fig. 104, 

 B). Under conditions severe enough to inhibit lateral, as well as dorsal, 

 lips, the developing blastopore takes the form of a transversely flattened 

 crescent (Fig. 104, C) and progresses subequatorially or even equatorially 

 around the embryo (Fig. 104, D). In these individuals the blastopore 

 makes its appearance near or at the equator, that is, much nearer the 

 apical pole than normally; and the progress over the yolk of the blasto- 

 pore lips may be completely inhibited, so that much or all of the basal 

 hemisphere remains a permanent yolk plug. In some gastrulae of this 

 type more or less elongation of the pigmented region in the apicobasal 

 axis may take place in recovery, but dorsiventrality and bilaterality are 

 apparently completely obHterated (Fig. 105, yl). These equatorial gastru- 

 lae have not been observed to develop appreciably farther, even after re- 

 turn to water; probably their failure to do so is associated with the more 

 or less complete obliteration through differential susceptibility of dorsi- 

 ventral and mediolateral gradient differences. However, a secondary in- 

 vagination often develops between the original blastopore and the apical 

 pole and may extend partly or completely around the embryo. Its char- 

 acteristics are similar to those of the primary invagination ; it begins dor- 

 sally and progresses ventrally (Fig. 105, B-D). The possibiHty that the 

 beginning of normal gastrulation involves some degree of physiological 



