628 The American Naturalist. [July, 
flat and single. The sponge grows out as a circular disk, later becom- 
ing irregular. It is covered by the flat ectodermal membrane and 
inside contains spicules all through the mesenchymatous substance of 
the body. 
The various canals and cavities of the sponge arise here and there 
with no arrangement. Later they connect with one another and 
break through to the surface as oscula and pores. The ciliated cham- 
bers are formed in the midst of special clusters of bulky mesoderm 
cells that divide to make walls about the intercellular space thus 
bounded. The way in which these special cells form the ciliated cham- 
bers varies in different larvae. 
In discussing the remarkable gemmule development the author 
points out that, if it has any value as indicating the past history of 
sponges, it is evidence of the former existence of a solid ancestor as 
maintained by Metschnikoff. It is mainly, however, the resemblance 
of this non-sexual larva to the egg larva of other sponges that is to 
be emphasized. Pointing out the resemblance in the formation of 
“germ layers” and the peculiarites of the anterior pole and changes 
of the “ectoderm,” Dr. Wilson then accentuates the comparison by 
applying certain views of Prof. Weismann. As any mesenchyme cell 
may, apparently, produce an ovum so any mesenchyme cell may unite 
with others to make a gemmule. The gemmule cell has, alone, but 
little histogenetic plasma, but an aggregate can form a larva. The 
gemmule cell is thus a germ-cell differing from the ovum in having its 
germ plasma not partly converted into ovogenetic plasma. Some such 
likeness between the egg cell and the gemmule cell is necessary to 
explain the observed resemblances between the egg larva and the 
gemmule larva. 
ARCHAEOLOGY AND ETHNOLOGY. 
MAN AND THE MYLODON. 
THEIR POSSIBLE CONTEMPORANEOUS EXISTENCE IN THE MISSISS- _ 
IPPI VALLEY. 
In one of the alcoves of the Museum of the Academy of Natural 
Sciences, Philadelphia is to be seen a considerable number of fossil 
bones of extinct animals belonging to the pleistocene period. In color, — 
texture and general outward appearance they have a remarkable sim 
1This department is edited by Dr. Thomas Wilson, of the Smithsonian Institution. | 
