1908-9. THE HABITS OF PLETHODON CINSREUS ERYTHRONOTUS. 477 
found moving slowly forward very nearly in the horizontal plane of the 
body; some few will be motionless and about 1% will be moving back- 
ward. It is not difficult to mark these latter by thrusting small splinters 
of .wood through the jelly until their points almost touch the sphere 
surrounding the larva; a few hours later these individuals will be found 
moving forward and another set moving backward. Reversal of direction 
has also been noted in some Plethodon larve. Evidently the condition 
in Plethodon is nothing peculiar to the genus but an exaggeration of 
one found elsewhere among Urodeles. How widely spread it may be 
and whether it means a reversing of the usual movements of the cilia 
~ are points left for future determination. 
The rate of rotation varies greatly, extremes of one minute and three 
and one half minutes having been noted under similar conditions. 
The service of the rotation to the embryo probably is that it prevents 
adhesion between the egg and the envelopes. Smith (’96) found this 
occurring in eggs of Cryptobranchus lying in dishes in the laboratory; 
interference with development resulted. He makes no mention of rotation 
in the eggs but suggests that gentle rocking due to the current of the stream 
in which the eggs are laid prevents adhesion under natural conditions. 
Apparently in eggs laid in waters that have no current—as Amblystoma 
or Rana—or in other equally quiet situation—as Plethodon—the rotation 
suffices to prevent adhesions until the embryo beginning muscular move- 
ments is past all danger of their occurrence. Besides rotation other 
movements in embryos of 6 to 7 mm. are performed at intervals and 
consist of elevating the head from the yolk and waving it from side to 
side. As development proceeds these movements pass into occasional 
wrigglings of the tightly coiled larva whereby the whole position within 
the envelope often becomes altered. 
Development and growth are very rapid for a few days, an embryo 
of 5 mm. will increase in length to 9 mm. in six days; but at this point 
the processes become much slower and to increase from 9 mm. to II 
mm. requires fourteen days. This change in the rate of growth is shown 
very plainly in sections by a sudden diminution in the number of mitotic 
figures which from being very numerous in the smaller embryos become 
rare after the length of 9 mm. is passed. In its later stages development 
is retarded in the central eggs of a cluster where the embryos are smaller 
and show but little reduction of the gills while the outer ones are larger 
and have the gills almost entirely absorbed (compare Figures 13 and 14). 
Measurements of a typical case gave central embryos 17.5 mm. with 
gills 1.75 mm. long and outer embryos 20 mm. with gills reduced to 
