278 AYERS ON THE DEVELOPMENT 



ovary. Within an imperfect lumen are seen several cells, larger than their fellows with peculiar bar-shaped 

 nucleoli. These are the germs of the ova. Fig. 4, X 300 ; fig. 5, X 250. 



Fig. 6. The relations of the fibres of the longitudinal and cross commissures of one of the thoracic seg- 

 ments at this stage. X 500. 



Fig. 7 Three pairs of large granular nuclei from the three thoracic ganglia of fig. 1. X 250. 



Fig. 8. Cross-section of an ab lo ninal ganglion showing a triangular remnant of the middle cord. The 

 two Seitenstriinge have not completely united. X 125. 



Fig. 9. A portion of the splanchnic inesoblast and its lining endoderm, from the region of the procto- 

 daeum of fig. 1. X 250. 



Fig. 10. Section of a fully formed dorsal vessel. The section is taken from the head region. X 250. 



Fig. 11. Section through the maxillary region of an embryo with the dorsal wall fully closed. X 125. 



Fig. 12. S.'Ction of the heart from the abdominal region. Two nuclei are seen in the walls of the vessel. 

 These are the nuclei of the two cells which compose the vessel in this section. X 250. 



Fig. 13. Section of an embryo after revolution, in the region of the third thoracic appendage. The gill 

 organ is cut through at its union with the body wall. The cavities extend the entire breadth of the organ 

 and are irregular both in size and in their course. X 125. 



Fig. 14. Section of an embryo passing through its gill organ. X 125. 



Fig. 15. Section of the oesophagus near its termination. X 150. 



Fig. 16. Section passing throttgh the commissural cords before their connection with the suboesophageal 

 ganglion. X 250. 



Fig. 17. A portion of the outer lobe of the right maxilla. X 250. 



Figs. 18-22. Successive sections of an embryo before revolution, passing through the proctodaeum and 

 through the flexures of the abdomen. Osmic-acetic acid preparation, stained in picro-earmine, mounted in 

 benzole-balsam. X 50. 



Fig. 23, 24. Sections of the same embryo through the first pair of maxillae and the antennae, respec- 

 tively. 



Fig. 25. Longitudinal section of an embryo. X 40. 



Fig. 26. Section through the abdominal region of this stage. X 125. 



Fig. 27. Transverse section of a germinal band of Oecanthus near the age shown in pi. 18, fig. 5. 



Fig. 28. Transverse section of the germinal band very soon after the appearance of the mesodermic cells. 

 X 100. 



PLATE XXIII. 



Fig. 1. Transverse section through the mandibular segment of an embryo after the closure of the dorsal 

 wall, showing the invaginations for the salivary gland and trachea of left side of this segment. X 125. 



Fig. 2. Section of another embryo between the mandibles and first maxilla. The section pusses through 

 the anterior p irt of the dorsal " plug " of the mesenteron, which has passed completely within the body at 

 this stage. On either side of the heart are seen two continuous tubes. X 125. 



Fig. 3. Transverse section through the supraoesophageal' ganglion. X 125. 



Fig. 4. Transverse section of the oesophagus in the maxillary region. X 125. 



Fig. 5. Transverse section of the abdomen passing through the proctodaeum at the point of origin of tha 

 three Malpighian tubi'S. These primitive tubes bifurcate soon after leaving the body of the proctodaeum. 

 X65. 



Fig. 6. An oblique transverse section of the the brain. X 125. 



Fig. 7. Amoeboid cells found in the yolk previous to the formation of the blastoderm. Some of 

 these cells go to the surface while others remain in the yolk an 1 form the endoderm and mesoderm. These 

 are taken from fig. 31, pi. 21. X 800. 



Fig. 8. Transverse section of a germinal band of about the same stage as fig. 3, pi. 19. The amnion is 

 seen on either side of the band as a few small cells closely pressed upon the ectoderm. X 800. 



Figs. 9-12. Sections through the thoracic and abdominal regions of the embryo to show the manner of 

 invagination of a median element to form a part of the nervous system. Osmic acid, balsam preparations. 

 Figs. 9 and 11 X 125 ; figs. 10 and 12 X 250. 



Fig. 13. Transverse section through the head at the stage represented in pi. 18, fig. 17, passing through 

 the oesophagus and upper lip. Osmic acid preparation. X 225. 



