Part 23

Fig. 1. Ovum, with primitive cumulus and streak proceeding from it.

Fig. 2. Somewhat later stage, in which the primitive cumulus is still visible. Near the opposite end of the blastoderm is a white area, which is probably the rudiment of the procephalic lobe.

Fig. 3a and 3b. View of an embryo from the ventral surface and from the side when six segments have become established.

Fig. 4. View of an embryo, ideally unrolled, when the first rudiments of the appendages become visible.

Fig. 5. Embryo ideally unrolled at the stage when all the appendages have become established.

Fig. 6. Somewhat older stage, when the limbs begin to be jointed. Viewed from the side.

Fig. 7. Later stage, viewed from the side.

Fig. 7a. Same embryo as fig. 7, ideally unrolled.

Figs. 8a and 8b. View from the ventral surface and from the side of an embryo, after the ventral flexure has considerably advanced.

Fig. 9. Somewhat older embryo, viewed from the ventral surface.

PLATES 31 AND 32.

COMPLETE LIST OF REFERENCE LETTERS.

ao. Aorta. ab.g. Abdominal nerve cord. ch. Cheliceræ. ch.g. Ganglion of cheliceræ. ep. Epiblast. hs. Hemispherical lobe of supra-oesophageal ganglion. ht. Heart. l.l. Lower lip. m. Muscles. me. Mesoblast. mes. Mesenteron. mp.g. Malpighian tube. ms. Mesoblastic somite. oe. OEsophagus. p.c. Pericardium. pd. Pedipalpi. pd.g. Ganglion of pedipalpi. pr. Proctodæum (rectum). pr.c. Primitive cumulus. s. Septum in abdomen. so. Somatopleure. sp. Splanchnopleure. st. Stomodæum. su. Suctorial apparatus. su.g. Supra-oesophageal ganglion. th. g. Thoracic ganglion. v.g. Ventral nerve cord. y.c. Cells derived from yolk. yk. Yolk. y.n. Nuclei of yolk cells.

Ig-IVg. Ganglia of ambulatory limbs. 1-16. Postoral segments.

Fig. 10. Section through an ovum, slightly younger than fig. 1. Shewing the primitive cumulus and the columnar character of the cells of one half of the blastoderm.

Fig. 11. Section through an embryo of the same age as fig. 2. Shewing the median thickening of the blastoderm.

Fig. 12. Transverse section through the ventral plate of a somewhat older embryo. Shewing the division of the ventral plate into epiblast and mesoblast.

Fig. 13. Section through the ventral plate of an embryo of the same age as fig. 3, shewing the division of the mesoblast of the ventral plate into two mesoblastic bands.

Fig. 14. Transverse section through an embryo of the same age as fig. 5, passing through an abdominal segment above and a thoracic segment below.

Fig. 15. Longitudinal section slightly to one side of the middle line through an embryo of the same age.

Fig. 16. Transverse section through the ventral plate in the thoracic region of an embryo of the same age as fig. 7.

Fig. 17. Transverse section through the procephalic lobes of an embryo of the same age. gr. Section of hemicircular groove in procephalic lobe.

Fig. 18. Transverse section through the thoracic region of an embryo of the same age as fig. 8.

Fig. 19. Section through the procephalic lobes of an embryo of the same age.

Fig. 20a, b, c, d, e. Five sections through an embryo of the same age as fig. 9. a and b are sections through the procephalic lobes, c through the front part of the thorax. d cuts transversely the posterior parts of the thorax, and longitudinally and horizontally the ventral surface of the abdomen. e cuts the posterior part of the abdomen longitudinally and horizontally, and shews the commencement of the mesenteron.

Fig. 21. Longitudinal and vertical section of an embryo of the same age. The section passes somewhat to one side of the middle line, and shews the structure of the nervous system.

Fig. 22. Transverse section through the dorsal part of the abdomen of an embryo of the same stage as fig. 9.

EXPLANATION OF PLATE 33. (XX. p. 714.)