72 T. H. MORGAN. 



REFERENCE LETTERS FOR PLATES. 



B. Brain. 



D. Mid-gut, mesenteron. 



D 1 , D", D ll \ D IV , etc. Diverticula of mid-gut. 



E. Ectoblast. 

 N. Entoblast. 

 M. Mesoblast. 



P. Periphery of blastoderm. 



S. Schizocoels. 



V l ~\ Ventral ganglia 1 — 7. 



H. Heart. 



L. Legs. 



0. Opening ventral organs. 



G. Central cavity of segmenting egg. 



DESCRIPTION OF PLATES. 



Plate I. 



Figure 1. Section of segmented egg and membranes of Pallene 

 empusa. 



Fig. 2. Section of another egg of Pallene at same stage to show 

 central cavity G. 



Fig. 3. Section through upper pole of egg of Pallene. First 

 formation of blastoderm. 



Fig. 4. Section through upper pole of egg of Pallene. Later 

 stage than last. Inner cells — entoblast — are present. 



Fig. 5. Section through egg of Pallene. Later stage than last. 

 Cap of cells — blastoderm — covers more of egg than in last. 



Fig. 6. Section through periphery of blastoderm, at P of Fig. 5. 



Fig. 7. Section in same region as last. 



Fig. 8. Section of segmenting egg of Tanystylum orbiculare, to 

 show pyramidal shape of cells. 



Fig. 9. Section of egg of Tanystylum, to show process of multi- 

 polar delamination. 



Fig. 10. Same, with delaminated entoblast. 



Fig. 11. Section of segmenting egg of Phoxichilidium maxillare. 



Fig. 12. Same, to show degeneration of endoderm. 



Fig. 13. Cross-section of the Pantopod -larva of Tanystylum 

 orbiculare. 



Fig. 14. Circumcesophageal ring of same. 



