PLATE 1 



EXPLANATIOX OF FIGURES 



11 Section through the anterior portion of the forebrain of a head-mero- 

 plast, showing unusual head-coelom. Total incubation thirty-two hours. Oper- 

 ation at the time of the first intersomitic groove (X 160). Experiment, Type 

 I, no. 19; b, anlage of ventral aorta; c, coelom; d, pharynx; e, forebrain. 



12 Section through the forebrain of a head-meroplast, showing early stages 

 in the formation of vasofactive cells. Total incubation, twenty-nine hours: 

 Operation previous to the formation of the first intersomitic groove (X 200). 

 Experiment, Type I, no. 24; a, prevascular mesenchyme; c, coelom; d, pharynx; 

 e, forebrain. 



13 Section through the forebrain of a head-meroplast showing a loose paren- 

 chyma in a position occupied by the isolated vasofactive cells of figure 4. Total 

 incubation thirty hours. Operation at the time of the first intersomitic groove 

 (X 150). Experiment, Type I, no. 18; a, prevascular mesench3rme; c, coelom; 

 d, pharynx; e, forebrain. 



132 



