1904.] NATURAL SCIENCES OF PHILADELPHIA, 755 



Fig. 11. — Pole view of second polar spindle. 



Fig. 12. — Outline of egg, sperm nucleus and expulsion of second polar body. 



Figs. 13, 14. — -Outline of eggs, second polar body on surface, egg nucleus at 

 right, sperm nucleus at left in both figures. 



Fig. 15. — Prophase of first cleavage, seven chromosomes in each nucleus; 

 sperm nucleus at left, egg nucleus at right ; yolk globules shaded. 



Fig. IG. — First cleavage spindle and second polar body. 



Figs. 17-19.— 2-cell, 4-cell and S-cell stages. 



Fig 20. — Optical section of a blastula. 



Fig. 21. — 16-cell stage. 



Figs. 22-24. — Successive gastrula stages, optical median section.-;. 



Figs. 25-27. — Optical median sections of later gastrulaj, showing the early 

 formation of mesenchym cells (shaded). 



Fig. 28. — Optical median section at stage of ectodermal thickening; mesen- 

 chym shaded. 



Fig. 29.— Optical median section at stage of beginning ectodermal invagina- 

 tion; mesenchym shaded. 



Figs. 30-32. — Similar views of successive later stages. 



Figs. 33-35.— Freehand drawings to show the armature of the proboscis in 

 extended position. 33, slightly oblique ventral view; 34, view from 

 the right side ; 35, slightly oblique dorsal view. 

 Plate L, Fig. 36. — Freehand drawing, on a much larger scale than on the preced- 

 ing plate, of a larva from the right side, with the proboscis fully 

 extended. 



Fig. 37. — ^Similar drawing of the proboscis in invaginated condition. 



