456 REPORT—1890. 
attached either to fixed foreign bodies, or to the male parents, or to each 
other by a more or less complicated transparent enveloping mass, and to 
see whether the attaching tissues in the different types are really different, 
or merely different modifications of an essentially similar secretion. 
Passing now from the Teleosteans, it is unnecessary to state how 
gladly I seized the opportunity here afforded me of studying the develop- 
ment of the Elasmobranchs. I might say of Pristiwrus melanostomus, for 
though I have obtained a few embryos of Torpedo marmoratus (mostly 
fairly advanced, with long external gill filaments) I have not yet examined 
these by sections, so that my work has been practically confined to the 
former type, of whose eggs I have received during my stay here a large 
number, notwithstanding the great demand there has been for this 
material from numerous other quarters; for this most graceful little dog- 
fish has, by reason of the diagrammatic simplicity of its development, the 
mournful satisfaction of being one of the biologists’ classic animals. 
The laid eggs, it appears, are never found, but the early stages 
can be obtained from the air-ducts throughout the year, and, as is 
known, these develop well if removed and placed in a tank where the 
sea water is slowly and constantly changed. It should be noted, how- 
ever, that while from January to the end of April I had practically no 
mortality among my eggs, as the warmer weather came on the death rate 
became considerable, and the conservator, Sig. Lobianco, informs me that 
his more extensive experience confirms this. Again, the development of 
this fish is slow, Pristiwrus, according to the above-mentioned authority, 
being about seven months old when hatched, so that, as none but the 
youngsters can be obtained by the fisherman, it requires time to rear the 
older ones. 
I have been fortunate enough to obtain a continuity of stages from 
A to O of Balfour, or from the first appearance of the segmentation cavity 
in the blastoderm to the time when the embryo is about an inch and a half 
in length, with long external gill filaments. From this material I ent and 
mounted, as soon as possible, a set of complete series of sections to guide 
me in my work. I have already found them very interesting and instruc- 
tive, and have reason to expect that they will be of great use in the 
future. 
Something may be said as to the methods I employed. The eggs of 
Pristiurus are, of course, only semitransparent, but by a proper adjustment 
of the light one can follow through the shell, using both reflected and 
transmitted light, all the changes in the blastoderm and embryo, such as 
the extension of the former over the yolk, and the origin and subsequent 
behaviour of the segmentation cavity, and the slow growth of the young 
fish. For examination of the living egg by reflected light, 7.e., viewing 
the blastoderm and embryo as opaque objects, sunlight, or even diffused 
daylight, may do; but for inspection as transparencies by transmitted 
light I could find nothing better than a simple candle flame held behind 
the egg, daylight, if present, being more or less screened off as necessary. 
s soon, then, as I received eggs of Pristiwrus I sketched each blasto- 
derm, and embryo if it had developed, as above described, and placed each 
egg by itself in a shallow glass vessel with a distinguishing number, this 
again being placed in one of the tanks of circulating water. The sketch- 
ing process was afterwards repeated, in the earlier stages daily and later 
at longer intervals. Perhaps Pristiurus is the only vertebrate (the shell of 
Scyllium would be too opaque) whose embryonic development can be 
ae 
