1892.] Embryology. 709 
the ventral wall of the gut, a paired mass of spindle-shaped mesoderm 
cells (each half of this mass of cells contains a cavity). This is the 
rudiment of the pericardium. It varies greatly in different individ- 
uals, but always has a paired origin. The two halves fuse later, but 
for a considerable time there remains a septum between the cavities of 
the two sides. The right cavity is from the first the larger of the two. 
Erlanger says: “ I was not able in Paludina to see the immediate 
transformation of the coelom into the pericardium, since the whole 
secondary body cavity is so early wholly filled with irregularly dis- 
posed, spindle-shaped mesoderm cells; yet the rudiment of the peri- 
cardium is formed between the two mesoderm layers, one of which 
clothes the inner surface of the ectoderm, the other the outer surface 
of the gut. 
“ The question then arises whether the pericardium represents the 
whole secondary body cavity (which would be greatly reduced) or 
merely a part of it, so that then the rest of the coelom would coincide 
with the primary body cavity or segmentation cavity. I now incline 
toward the second view, and think that the coelom only partially per- 
sists as such in the pericardium, while by far the greater part of it is 
obscured by the spindle-cells which fill it and so simulates the primary 
body cavity [—und daher sich mit der primären Leibeshéhle deckt]. 
The development of Paludina, described in this paper, appears to me 
to uphold this conclusion, and my other not yet completed researches 
in regard to the manner of formation of the blood-vessels strengthens 
me in this view.” 
The kidneys arise as evaginations, right and left, of the pericardial 
wall, while the embryo is still untwisted. They are first indicated by 
thickened areas of this wall. These thickened areas push out toward 
the mouth chamber until they assume a tubular form. The rudiments 
of the renal ducts arise at the same time with the latter, as evagina- 
tions, right and left of the walls of the mantle chamber, toward the 
rudiments of the kidneys. The right kidney rudiment and the rudi- - 
ment of the right renal duct unite to form the permanent kidney. 
The left kidney is never fully formed, its rudiment never uniting with 
that of the left renal duct. Each of these persist for a time, but 
during the subsequent spiral twisting of the embryo each is obliter- 
ated. The secretory portion of the kidney arises then, from mesoderm 
and not from ectoderm, as has been claimed. Its excretory duct arises 
from the ectoderm of the mantle chamber. 
The heart arises as an invagination of the dorsal area of the peri- 
cardium, forming an antero-posterior furrow. Soon this furrow con- 
