274 AYERS ON THE DEVELOPMENT 



p. primitive unpaired organ (metastomum). ?_. pleural structure (gill-pad). 



A. dorsal organ. p. cup-shaped organ. 



PLATE XVIII. 



Fie. 1. The youngest observed germinal band of Oecanthus niveus. The serosa is not yet formed. 

 X 25. (The egg is magnified only 15 diameters.) 



Fie. 2. Longitudinal optical section (diagrammatic) of fig. 1. 



Fig. 3. The germinal band after the appearance of the head fold, which is indicated at this time by the 

 more rapid growth and consequent greater breadth of the lower end of the embryo. X 2o. 



Fig. 4. A young embryo of Oecanthus after the appearance of the primitive segment folds. X 50. 



Fig. 5. A more advance 1 embryo, with the anteunal folds distinctly marked off. The free ends of 

 the primitive folds have united across the embryo posterior to the anteunal folds. X 50. 



Fie. G. Deep, or primitively head, end of the egg after the formation of the serosa. X 25. 



Fig. 7. Upper lip and antenna of a somewhat older embryo. X 100. 



Fi<rs. 8, 9. Ventral and side views of the embryo with the appendages sprouting out. X 25. In its natural 

 con litions the embryo, having reached this stage of development, remains dormant for six months on account 

 of the cold of winter. 



Fie. 10. Ventral view of the tip of the ab lomen of the stage figured in fig. 13. The large cells of the 

 amnion cover part of the embryo. X 50. 



Fi' r . 11. Ventral view of another embryo. Acetic acid carmine preparation, x 50. 



Fio-s. 12, 13. Lateral and ventral views of an embryo more advanced than that shown in figs. 8 and 11. 



X50! 



Fie 14. Camera outline of the head region of the stage represented in fig. 11. Shading diagram- 

 matic. Xl25. 



Fie. 15. Head of an embryo, somewhat older than fig. 8, in which the invagination at the base of the 

 antennae to form the internal skeleton of the head is well advanced. Acetic acid, carmine, glycerine 

 preparation, x 125. 



Fie. 16. A portion of the oesophagus near the mouth showing the salivary ducts united into a common 

 tube. X 65. 



Figs. 17, 19. Ventral views of two embryos of nearly the same age. Chromic acid preparations, x 50. 



Fig. 18. Side view of another embryo of about this stage. X 65. 



Fif. 20. An embryo with the abdominal flexures straightened out. Chromic acid preparation. X 50. 



Fif. 21. The appearance of the embryo in ventral view just before revolution. Sodium chloride 

 preparation. X 50. 



Fie. 22. An embryo in which the lobation of the mouth parts has begun. The abdominal region is 

 curved upon itself while the proctodaeum forms another curve, projecting into the yolk. The amnion 

 extends as far as the anal stylets. Osmic acid preparation. X 65. 



Fie. 23. Lateral view of the head, maxillary and thoracic regions of another embryo of about this stage- 

 Chromic acid preparation. X 75. 



Fie. 24. Optical section of an embryo with three folds of the ectoderm projecting into the head cavity 

 These thickenings lie at different depths, respectively 0, 1, 2. X 75. 



Fi<>. 25. Dorsal view of the end ot the abdomen of an individual showing the projecting tips of the a. s 



X 50. 



Fig. 26. An oblique dorsal view of the proctodaeum of a younger individual. X 50. 



Fie. 27. Ventral view of the abdomen and proctodaeum of an embryo. X 75. 



Fie. 28. The "gill-pad" structure, /., highly magnified ; partly diagrammatic. X 225. 



Fie. 29. Optical section of the end of the abdomen ami proctodaeum of an embryo before revolution. 



X 65. 

 Fie. 30. The tips of a foot and an antenna, showing the cuticula as a loose sac enclosing the appendage. 



X 50. 



