1892.] Embryology. 627 
development of certain undescribed silicious sponges, found upon the 
Massachusetts Coast and in the Bahama Islands. 
Though the egg development was studied in a number of sponges it 
is only the non-sexual development by gemmules that is of special 
interest in this connection, 
In Esperella fibrexilis n. sp. of Woods Holl, Mass., the first appear- 
ance of the gemmules was traced to certain plump cells in the meso- 
derm. Such cells collect into groups of varying size: in each group 
the central ones are rather closely packed while the outer ones arrange 
themselves in the form of a follicle. Such clusters are the gemmules. 
Though it is possible that some gemmules may be formed from single 
cells, most are aggregates of many separate cells and in all growth 
takes place not only by cell division inside the gemmule, but by the 
actual fusion of large and small gemmules; so that the ultimate mass 
is of complex multicellular origin. 
When the gemmule is full grown it forms a spherical mass of closely 
packed cells with faintly marked boundaries and full of yolk. The 
entire mass now projects into a water-canal, suspended as it were by 
the stalk-like attachments of the follicle. 
In this ripe gemmule a remarkable process of subdivision now takes 
place. The solid aggregate of cells breaks down into clusters of cells, 
which are separated by liquid. The division continues until all the 
cells of the gemmule became separated from one another. Then the 
outer ones rearrange themselves to form an outer layer covering a 
central mass of ameeboid cells, all connected together by processes, 
though separated by liquid. 
The outer layer becomes ciliated and rich in orange colored pig- 
ment. At one pole, however, this change does not take place and at 
the same pole the inner cells crowd together to form a dense mass in 
which spicules appear. 
In this condition the larva breaks out of its follicle, leaves the 
sponge and swims about actively in the water. 
The gemmule larva thus closely resembles an egg larva in having 
an outer layer of pigmented, ciliated cells (ectoblast) replaced at the 
anterior pole by a thin layer of cells not pigmented nor ciliated ; while 
the mesenchymatous central mass is dense and has spicules only at this 
same anterior end. 
The larva attaches itself by its anterior end, but obliquely, so as to 
lie upon its side. Even before this a change extended from the ante- 
rior pole over the rest of the larva, the ectoderm becomes gradually 
‘Journal of Morphology, V, 1891. 
