PLATE 1 



EXPLANATION OF FIGURES 



1 Late gastrula or early bipinnaria stage showing mesenchyme migrating 

 from the apical plate (ap) and coming in contact with the anterior vesicle (av). 

 The archenteron bears the usual two enterocele pouches and a single left poste- 

 rior one. 



2 Anterior end of bipinnaria showing the completion of the mesenchyme 

 formation at the apical pole and its migration to the esophageal region. The 

 left hydrocele is provided with pore canal and hydropore. Ip left posterior 

 vesicle. 



3 Section of gastrula showing apical plate with signs of mesenchyme 

 formation. 



4 Bipinnaria showing mesenchjTne migrating past the anterior vesicle (av) 

 to the esophagus. The left hydrocele has severed its connection with the archen- 

 teron. Ip left posterior vesicle in process of formation. 



5 Surface view of gastrula immediately before mesenchyme development 

 from apical plate. 



