REPRODUCTION IN THE UNIONID^ 
113 
some of the difficulties involved in the investigation, which is 
still in progress, may be overcome. 
The histological changes taking place in the epithelium during 
the formation of the cyst have been s'^udied in this laboratory by 
Miss Daisy Young, and as her results will soon be pubhshed in 
detail, only a very brief reference will be made to the subject 
here. We are indebted to her for the use of six of her drawings 
which are reproduced in this paper in figs. 33-38, in order to 
illustrate the essential points involved in the cellular changes oc- 
curring during implantation. 
Fig. 37 shows an early stage, 6 hours after attachment, in the 
formation of the cyst on the fin. The proliferation begins in the 
neighborhood of the constriction where several mitoses may be 
seen in the figure, and this seems to be the region of active growth 
and multiplication of cells. As the cells at this level mcrease in 
number, they appear to push those lying above them up over the 
outside of the shell, so that the actual covering of the glochidium 
is due largely to this mechanical gliding of the epithelium over 
its surface. Sections give no evidence at all of amitotic division, 
while mitoses are generally abundant in the region of active pro- 
liferation. Fig. 38 shows a case of complete implantation on 
a fin in 24 hours. The wall of the cyst is seen at this time to 
be quite thick, but it usually becomes thinner later on as the cells 
composing it flatten down. 
In figs. 34, 35, and 36, a series of stages are represented in the 
formation of the cyst on gill-filaments, taken at 15 minutes, 30 
minutes, and 3 hours, respectively, after attachment. Fig. 34 
shows the very beginning of the prohferation and the presence 
of two or three mitotic figures just below the glochidium and near 
the raw edge of the constricted epithelium. A little coagulated 
blood is seen on the surface of the shell and around the lips of the 
wound, showing how intim.ately the larva com.es into contact 
with blood at the time of attachment, as referred to above. A 
large mass of the fish's tissues, including portions of blood ves- 
sels, is also shown in the figure enclosed within the mantle cham- 
ber. At the next stage (fig. 35), the pushing-up of the epithelium 
has m.ade considerable progress; several mitoses appear in the 
THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. .9, NO. 1. 
