116 



GASTRULATION 



exact patterned arrangement, they take their course according to independ- 

 ent formative tendencies which originate in the parts themselves. Thus we 

 find in the gastrula stage a mosaic, a pattern of parts with definite formative 

 tendencies from which the formative tendencies of the whole must neces- 

 sarily follow. 



Gastrulation, according to Spemann, is an intricately arranged mosaic 

 of parts possessing autonomous movement potentials which act in 

 the right way at the right time. 



Animal hemisphere 



Blastocoel 



Vegetal hemisphere 



Dorsal blastopore lip 



Yolk endoderm . . , 



Archenteron 



Blastocoe 



Archenteron 



Completion 

 bridge 



Blastocoel 



Endoderm Notochord 



Ectoderm 



Yolk plug 

 Ventral blastopore lip 



Gastrol ,,p*<sssaas^sy Lateral 

 mesoderm / blastopore 



Yolk plug ''P 



Blastula to gastrula stages in the Irog. Shown by stereograms. 



{Continued on f acini,' page.) 



