PHYSIOLOGICAL CHARACTERISTICS OF AXIATE PATTERNS 137 



THE GASTRULA AND LATER STAGES 



The dye-reduction gradients in early echinoid gastrulae are essentially 

 similar to those of late blastulae. The mesenchyme in the blastocoel re- 

 duces earlier than any other cells. The entoderm follows closely; and as it 

 elongates inward, it develops a distinct basipetal gradient, that is, the 

 radial differential present before invagination becomes longitudinal with 

 invagination, as in a bud. In the ectoderm rate of reduction decreases 

 from both apical and basal poles and from the ventral side dorsally (Fig. 

 49, .4). Later the beginning of development of the stomodeum and of the 

 oral lobe from the apical region is accompanied by increased rate of reduc- 

 tion in those regions, and rate of reduction ventrally is distinctly higher 

 than dorsally (Fig. 49, B). 



As the pluteus form begins to develop, the oral lobe and the region of 

 each anal arm show reduction, decreasing in rate from the tips. Elonga- 

 tion of the dorsal region is also accompanied by a slight reduction gradi- 

 ent, with high end at the tip. With completion of pluteus development 

 the reduction gradients become less distinct : changes associated with met- 

 amorphosis have not been observed. 



In the early gastrula of Patina the invaginating entoderm does not 

 show a high rate of reduction at its tip, but entodermal rate is highest at 

 the blastopore, decreasing in both directions; in the ectoderm, rate de- 

 creases from apical and basal regions (Fig. 49, C). As enlargement and 

 decrease in thickness of the inner free end of the entoderm begins, a new 

 reduction gradient with high end at the tip appears, that is, the basipetal 

 gradient appears here, but later than in the echinoid (Fig. 49, D). That 

 the difference is in some way associated with formation and isolation from 

 the epithelial layer of mesenchyme preceding gastrulation in the echinoid 

 and at a considerably later stage (after invagination in the starfish) seems 

 probable. 



As decrease in thickness of the apical end of the entoderm progresses, 

 with separation of mesenchyme cells into the blastocoel and development 

 of the esophagus, the basipetal gradient becomes still more distinct (Fig. 

 49, E). The region about the blastopore, however, still shows early reduc- 

 tion, perhaps associated with contractile activity there, which is evident 

 at this stage in closure and opening of the blastopore, as if a sphincter 

 were present. The beginning stomodeal invagination is represented by a 

 locaHzed radial gradient system or held of rapid reduction, and the whole 

 ventral side of the larva reduces more rapidly than the dorsal (Fig. 49, E). 

 As the ciliated bands develop, they reduce somewhat earlier than the 



