Moseley's remarks bearing on zooxanthellae
[Space here for quoted text from Moseley]
Certain remarks of Mangan
A large section of Mangan's paper dealing with development of the egg and zooxanthellae:
In the material examined it was clear that the vacuolated substance of the ova in the free medusae, in all cases, contained numerous zooxanthellae; but they were equally absent from the ova of the numerous unliberated medusas that had been inspected.
However, after a long search some half a dozen medusae were found which, I think, afforded conclusive evidence that zooxanthellae pass in numbers from the manubrium into the ovum, and that this invasion may commence at a period prior to the liberation of the medusae.
The ovum, until it has attained a considerable size, is sharply
divided off from the vacuolated substance of the manubrium.
Eventually a period is reached when its compact cytoplasm becomes continuous with the central mesh work (fig. 8). This stage could be commonly observed, and zooxanthellae be seen round the margin of the egg, but never in its compact substance. Sometimes the egg cytoplasm displayed small incipient vacuoles.
The subsequent period, during which vacuolisation of the cytoplasm of the ovum occurs, and during which the zooxanthellae are incorporated, could be found only on one piece of Millepora, where half a dozen medusae had reached this stage (fig. 9). Their ova showed a variable amount of vacuolisation, which, so far as I could make out, is in great part inaugurated at the inner borders of the egg substance; as if in some manner the vacuolated portion of the manubrinm with its contained zooxanthellas were drawn in.
That the medusas are liberated at this stage can be concluded from
their mature appearance within the ampullae, and from the fact
that one free medusa (fig. 10) was met with which exhibited an
essentially similar structure.
The other free medusas examined, possessed ova, more completely
vacuolated, and with more numerous zooxanthellae. The egg-cells
showed various phases of an encroachment upon the manubrial
substance, which eventually was almost entirely reduced (fig. 12),
the ova at the same time becoming rounded off and similar to the
extruded ova that were examined.
During all this period the zooxanthellae exhibit their normal appearance, and it can be observed that they divide fairly frequently within the ovum. These cells have been figured by Moseley ('81) who was able to examine fresh material. He remarks that they closely resembled those of other Hydroids. They contained irregular granules of a bright gamboge-yellow colour,
gamboge
n 1: a gum resin used as a yellow pigment and a purgative;
2: a strong yellow color [syn: {gamboge}, {lemon}, {lemon
yellow}, {maize}]the cell-contents frequently dividing into two, and sometimes, more rarely, into four (emphasis mine, AED). In the older portions of the colony the pigment; was of dark-brown hue.
I show their structure as displayed in stained specimens (fig. 18). The spherical nucleus exhibited a mass of closely packed chromatin granules. A pyrenoid was always present, the clear space around which, in most cases, gave the reaction for starch. The pigment-bearing granules varied in number and size, did not always stain to the same degree, and in some cases had a little starch associated with them. The cell membrane did not respond to cellulose tests. I observed in a few cases division of a cell into four. Their average diameter was
somewhat over 9um.
That zooxanthellae pass into the ovum from the parental tissues
appears to be undoubtedly the case from the foregoing evidence;
but the part these play in the future economy of the animal
remains to be solved. It may be that their enclosure is more
accidental than physiologically necessary, for the bulk of the
foreign cells in the canals and tissues of the colony may come
from the surrounding water at a subsequent period. An apparent
increase in the substance of a mature ovum (see below) might be an
indication of their activity. At all events, there is suggested,
an approach to a more complete symbiotic union than that which
exists in Convoluta roscoffonsis, for instance, where ifc has been
definitely shown ('07) that infection takes place after the animal
is hatched, for there the animal undoubtedly plays, in the long
run, the part of a parasite with respect to the alga, as under no
known conditions were algte found to escape alive from the body of
that turbellarian.
No comments:
Post a Comment