492 



EIGENMANJST. 



Abbreviations. 



n. r. neural ridge. 



op. optic stalk. 



pc. pericardium. 



pr. ec. primitive ectoderm. 



pr. ent. primitive entoderm. 



s. d. segmental duct. 



s. c. sex-cells. 



seg. c. segmentation cavity. 



so. mes. somatic mesoblast. 



sp. splanchnic mesoblast. 



yk. yolk. 



EXPLANATION OF PLATE XXXI. 



All the figures are made with an Abbe camera and Zeiss D and 4. X 355. 



Fig. 1. Part of a section nearly tangential to the yolk of an egg shortly before 

 the closing of the blastopore. Osmic chromic acetic. Grenadier's haematoxylin. 



Fig. 2. Part of a section of an egg shortly before the closing of the blastopore. 

 Osmic chromic acetic. Grenacher's alcoholic borax carmine. 



Fig. 3. Cross-section of the anterior part of the embryonic axis before any pro- 

 tovertebrae are outlined. There are two sex-cells in front of this section, five just 

 behind it, and two farther back. Osmic chromic acetic. Grenacher's haematoxylin. 



Fig. 4. Section parallel to the tangential over the head of a similar embryo, 

 and from the same ovary as Fig. 3. 



Fig. 4 a. Outline of the next section. 



Fig. 5. Section of the head parallel to the sagittal of a larva, with two well-formed 

 protovertebrse, showing sex-cell in auditory thickening (?). Osmic chromic acetic. 

 Delafeld's haematoxylin. 



Fig. 6. Another section of the same embryo, showing the group of sex-cells just 

 in front of the first protovertebra. 



Fig. 7. Section through the head of a larva 1.1 mm. long, showing position of 

 sex-cells. The section is oblique, the auditory vesicle of one side showing and the 

 optic stalk of the other. The middle of the figure is median to the larva, as is seen 

 by the neural canal, n. c. Grenacher's alcoholic borax carmine. 



Fig. 8. Cross-section through the anterior part of the anus of another larva from 

 the same ovary, showing the group of sex-cells in the as yet united somatic and 

 splanchnic mesoderm. 



Fig. 9. Cross-section through the corresponding region of a larva 2.5 mm. long. 

 Osmic chromic acetic. Grenacher's hsematoxylin. 



FlG. 10. Another section of the same larva further formed. 



Fig. 11. Part of the blastoderm of an egg containing about seventeen hundred 

 cells, large sex-cells, and multinucleated cells. 



