800 SUMMARY OF CURRENT RESEARCHES RELATING TO 
that the tendency of those who, in the future, examine the structure 
of both the hard and soft parts together will be rather to diminish 
than increase the number of existing species. 
Some of the specimens in the present collection have been so 
carefully preserved as to bear, with excellent results, the most elabo- 
rate histological treatment. The structure of the soft parts of Stylaster 
gracilis agree very closely with that of S. densicaulis, as described 
by the late Prof. Moseley, but there are usually twelve tentacles on 
the gastrozooids, instead of eight. Nematocysts appear to be confined 
to the tips of the dactylozooids. The muscular slip of the dactylo- 
zooid consists of a thin sheath of ectoderm covering a bundle of 
parallel muscular fibres, which are doubtless of ectodermic origin ; if so, 
the endoderm is entirely obliterated in this “ slip.” The ripe eggs 
are 0*25 mm. in diameter, and are, therefore, considerably smaller 
than the eggs of Distichopora or Allopora ; the early stages of the 
developing egg resemble those already described by the author for 
the two just-mentioned genera. It is thought possible that Stylaster 
has been derived from a form closely resembling Allopora bv the 
terminal branches remaining more delicate, and consequently, showing 
more definitely than the ancestral type the regular alternate method 
of gemmation. 
The marked differences in colour presented by different specimens 
of Distichopora violacea are due to differences in age and sexual 
condition. Some details are given as to the histology of the two 
kinds of zooids of this species, and it is remarked that the mesoglcea 
is much more extensive than in Millepora ; the skeleton is not in close 
contact with the ectodermal cells in the deep parts of the corallum, 
but lies imbedded in the mesogloea. In Millepora , on the other hand, 
the mesogloea is represented by a very thin perfectly structureless 
lamella, which is situated between the ectoderm and endoderm, and 
the skeleton lies in all cases outside the ectoderm, in close contact 
with its cells. 
In conclusion it is pointed out that some problems must remain 
unsolved till well-preserved specimens of Millepora have been re- 
examined. 
Formation of Germinal Layers in Hydromedusse.* — Herr W. Gerd 
reports that he has observed in Bougainvillea an equal division into 
two and four blastomeres, and a coeloblastula, the fate of which was, 
however, different from that described by Metschnikoff. In this form 
the cells divide in tangential directions, and increase in numbers 
while the nuclei take up a peripheral position, they elongate in the 
radial direction, and so diminish the cavity of the coeloblastula. In 
the course of further cell-multiplication the boundaries of the cells 
become indistinct. Later on, there is a migration of cells into the 
interior of the coeloblastula ; there is a multipolar migration which 
leads to the formation of a compact morula in which the peripheral 
and central nuclei are altogether identical. As in the case of Tubu- 
laria , described by Brauer, the result of migration is not a planula, 
but a morula. 
Zool. Anzeig., xv. (1892) pp, 312-6. 
