NOTES ON THE EMBRYOLOGY OF LIMULUS. 533 
dages, and eventually join the dorsal mesoblastic bands men- 
tioned a few lines above. The septa are not wholly formed 
from these ventral ingrowths, but at the same time lateral 
inpushings are taking place as shown in fig. 13, the section 
being horizontal and passing above the level of the eyes, so 
that only a few of the abdominal segments are included. The 
result of this is that the yolk in the cephalothorax is broken up 
into a central mass and six pairs of lateral lobes, the history of 
which will be traced later. 
It is not an easy task to trace the history of the coelom past 
the point where we left it, though some isolated features are 
readily seen. Thus it is seen that the mesoblast does not 
split in the dorsal region until after the formation of the heart 
as a tubular organ. 
Heart. 
Soon after the union of the two halves of the mesoblast in 
the dorsal region a longitudinal cord several cells in thickness 
is formed. How this thickening is produced I cannot say. 
In the yolk beneath it the nuclei are very numerous, and the 
cells are much smaller than in other parts, and it may be that 
some of these migrate into the mesoblastic tissues ; but although 
I have examined many sections I have not yet seen any indis- 
putable evidence of such migration. 
Soon a lumen appears in this cord, and the size increases, at 
least to a considerable extent, through the budding of mesoblast 
cells into the tube, where they become transformed into blood- 
corpuscles. These processes are represented in figs. 31 to 33, 
which represent sections from the same individual, 31 being 
the most posterior. Two sections back of this is the end of 
the lumen. Between 31 and 32 intervenes a distance of 0'025 
mm., and between 32 and 33 a distance of 02 mm. From 
this it appears that the heart is formed from in front back- 
wards, and gradually the walls are reduced to a single cell in 
thickness. Not until after this stage is reached does the heart 
separate from either of the mesoblastic layers, and from these, 
first from the splanchnoplure (fig. 33). This single-celled 
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