602 WILLIAM PATTEN. 



Fig. 30. — Cross section of the larva, showing the fully formed heart, hb., 

 and the wall of the raesenteron. 



Fig. 31. — Part of a section through a larva, a, segmental thickening of 

 the hypodermis opposite the tracheal trunk, tr. 



Fig. 32. — Section through dorsal region of an advanced embryo, showing 

 thickened cord of cells from which the heart is to be formed, e. n. yolk-cells, 

 attaching themselves to the wall of the yolk-sac to form the mesenteron. 



Figs. 33, 34, and 35. — Very highly magnified cells of the ventral plate, 

 showing various kinds of nuclei. 



Fig. 36. — Cross section of the embryo through the first pair of maxilla;, 

 showing the tracheal invaginations ; at a are very large endoderm cells. 



Fig. 37. — Sagittal section of the three thoracic ganglia, showing the double 

 cross commissures. 



Fig. 38. — A longitudinal section of the same ganglia, showing the longi- 

 tudinal and double cross commissures. 



Fig. 39. — Longitudinal section of the dorsal organ of an embryo shown in 

 PI. XXXVIa, 6g. 15. 



Figs. 40 and 42. — Successive stages in the disappearance of the dorsal 



