1908-9. ] THe Hasits OF PLETHODON CINEREUS ERYTHRONOTUS. 483 
teduced to a few small points not over .25 mm. long. Sections of several 
larve of this general stage but differing slightly in development show 
that both the gill slits become closed before escape from the envelopes 
occurs, the second one being the first to close. 
In their development the digits of the posterior limb but partly 
bear out the expectations of Cope (’89). He would regard Hemidactylium 
with its posterior foot possessing only the first four digits as having for 
permanent form that which is larval in Plethodon. Dealing with P. 
cinereus he finds the adult has the outer digit longer than the inner, but 
in younger specimens it is shorter and in his youngest (18 mm. only 
but having already lost its gills) it is but a minute tubercle, ‘‘and in a 
little earlier stage cannot but be wanting though this I have not seen.” 
On this point fifty-one larve were examined covering twelve stages, begin- 
ning with larve of 11 mm. long and ending with the smallest found living 
alone, its length being 23 mm. In larve of 12 mm. the whole five digits 
appear at once and the fifth is no less prominent than the first. There 
follows a brief period—vz. until the larva attainsa length of 16 mm.— 
in which the rate of growth of the fifth digit as compared with that of 
the first, varies; of twenty-nine specimens within these limits the first 
exceeded the fifth in sixteen, the fifth exceeded the first in three, and in 
ten they were equal. (Seven broods are represented in this, in four of them 
only were all the larve alike on this point). This period alone is in accord 
with the argument of Cope. In all larve over 16 mm. (thirteen in number 
representing four stages) the external toe was longer than the internal. 
EFFECTS OF TERRESTRIAL DEVELOPMENT. 
The influence of a purely terrestrial development is seen chiefly in 
the following points. 
The large amount of yolk in the egg in proportion to the size of the 
animal, a point already dealt with. 
The yolk mass retains its globular form until late in development 
(larve of 13 to 15 mm.), when the absorption of its substance causes 
it to become fusiform. In aquatic larve the mass early elongates to 
produce a slender body capable of rapid darting movements, a necessity 
not laid upon the yolk in the inactive larva of Plethodon. 
The development of limbs occurs early. Traces of the anterior 
limbs are distinct in larve of 5 mm. and of the posterior limbs in those 
of 6 mm. In Amblystoma these traces appear in larve of 7.5 mm. 
and 13 mm. respectively. From the large size of the posterior limbs 
