NOTES ON THE EMBRYOLOGY OF LIMULUS. 
575 
PLATE XXXVIII. 
Fig. 21. — Transverse section through fourth segment of embryo, shown in 
Fig. 12. The mesoblast has met above and below. X 64. 
Fig. 22. — Longitudinal section through the fourth to the seventh segments 
of an embryo in stage shown in Fig. 12. The section passes to one side of 
the median line, and shows the metastoma as a portion of the sternum of the 
sixth segment. X 76. 
Fig. 23. — Longitudinal section of embryo, Fig. 12, showing the opening of 
the segmental organ and the vesicular enlargement, x 150. 
Fig. 24. — Section through the nervous cord (not yet separated from the 
epiblast) behind the eighth ganglion of Fig. 12. x 380. 
Fig. 25. — Section through seventh ganglion of Fig. 12. The separation of 
the nervous tissue from the epiblast is complete in the middle, though not at 
the sides. 
Fig. 26. — Section parallel to and outside of Fig. 23, cutting through three 
folds of the segmental organ. A thirteenth of a millimetre intervenes between 
the two sections. 
Fig. 27. — Transverse section of half of fifth segment, showing three 
portions of the segmental organ and its internal opening,/®, x 150. 
Fig. 28. — Section of middle portion of segmental organ, x 1000. 
Fig. 29. — Median sagittal section of embryo. Fig. 15. The section passes 
through the edge of the mouth (mo) and shows the epiblastic involution (ei) 
which later takes part in the formation of the brain. Though the first pair of 
appendages are distinctly preoral (cf. Fig. 12), the first ganglion of the ventral 
chain ( gn i) retains a postoral position. Behind the eighth ganglion the 
nervous tissue has not separated from the epiblast. X 75. 
PLATE XXXIX. 
Fig. 30. — Transverse section of third appendage and adjoining region of 
Fig. 6 at the first appearance of the coelom, showing it to be a schizocoele. 
X 150. 
Figs. 31 — 33. — Formation of the heart, Fig. 31 being the most posterior. 
A twentieth of a millimetre behind Fig. 31 no lumen occurs in the solid 
mesoblast. A thirteenth of a millimetre intervenes between Figs. 31 and 32, 
and a fifth of a millimetre between Figs. 32 and 33. The conversion of 
mesoblast cells into blood-corpuscles is shown in all three figures. The 
thickening of the dorsal epiblast immediately above the heart is noticeable. 
X 442. 
Fig. 34. — Constructed from three longitudinal sections of Fig. 6, showing 
portions of ihe coelom in each segment of the body. 
