710 The American Naturalist. [August, 
stricts in the middle, indicating the line of demarkation between auri- 
cle and ventricle. The heart furrow sinks more and more into the 
pericardial sac until it becomes a closed tube distinct from the latter, 
except at its two openings, one on the originally anterior, the other on 
the originally posterior face of the pericardium. At the same time 
art has divided into an auricle (posterior) and a ventricle 
(anterior), as was before indicated by the constriction in the heart — 
furrow. 
By the twisting of the body of the embryo the heart is brought upon 
the left side and the kidney to the middle line. The kidney grows 
larger, its opening into the pericardium becomes narrower, and the 
external orifice of the renal duct grows smaller and smaller. At the 
same time the cells of the wall of the kidney enlarge and in different 
places the walls push into the lumen of the gland, forming strands 
which by further development are converted into a mass of spongy 
tissue. The kidney, then, is not a typical acinous gland. 
The primitive kidneys (“ Urnieren”) arise from mesoderm, one on 
each side of the embryo, just behind the velum. They first appear 
at the time of the stomadeal invagination, while the embryo is still 
wholly symmetrical. Each rudiment is at first a solid mass of cells. 
Soon a cavity appears within the mass, and at the same time it 
approaches the surface. It soon breaks through the ectoderm cells to 
the surface. Its cells can be distinguished from the ectoderm and the 
rest of the mesoderm by their large size, clear protoplasm and deeper 
staining. It is still a closed vesicle. Now it elongates, becoming 
tubular, and soon it gains an external opening. There is no inte 
opening for the primitive kidney. Its inner end is formed by a mass 
of spindle-shaped and star-shaped mesederm cells, at least one of which 
bears long cilia, which are active in the living embryo. No concre- 
tions or excretory granules are present. In the absence of any inter- 
nal opening the primitive kidney of Paludina resembles the excretory 
organs of Plathelminths, “ yet it may be possible that this departure — 
from the ordinary condition is only the result of a certain degenera- 
tion.” Erlanger thinks that in the pair of primitive kidneys and the 
pair of permanent kidneys (only one of which fully develops) we may 
have represented the segmental organs of two segments, comparable to 
the segmental organs of the worms. 
The author shows that each of the ganglia of the nervous system 
arises by a sort of delamination from thickened areas of the ectoderm. 
All but the visceral ganglion arise fom paired rudiments. The vis- 
