1908-9. | THE HABITS OF PLETHODON CINEREUS ERYTHRONOTUS. 479 
bunch being kept together and separate from other bunches. In the series 
twenty different stages of development are represented, though only the 
most important of them will be described. As the eggs vary somewhat 
in size so do the individuals of the same stage of development; the 
descriptions given are those of average specimens, extremes in any 
direction (very few in number) being disregarded. 
The embryo may be considered as well defined upon the yolk when 
the medullary folds are complete and approximated through a little more 
than the posterior half of their extent. (Figure 4). Together they give 
this part of the embryo a width of .75 mm. except at the posterior end 
where they separate slightly before becoming continuous giving here a 
width of 1 mm. In their anterior part the folds are widely separated 
giving to that part of the head an extreme breadth of 1.5 mm. In front 
of this part an almost transverse piece connects them while behind it, 
they converge, meeting at a point a little anterior to the middle of the 
body. In living specimens placed in a strong light a delicate scalloping 
of the inner margins of the folds in the cranial region indicates clearly 
the neuromeres, but no method yet used has succeeded in preserving them 
through fixation. In a corresponding stage in the eggs of some Amphibia 
the areas of brain substance that will form the optic vesicles are indicated 
by depressions, sometimes pigmented (Eycleshymer ’93), but in Plethodon 
no trace of these can be found. ‘The length of the figure defined by the 
medullary folds is three millimetres. 
Larva 5mm. Figure 5. 
The form of the body is still determined by the yolk and the central 
nervous system. The medullary folds are in apposition throughout. 
The head is raised off the yolk as far back as the midbrain; eyes and ears 
are indicated externally. Behind the head the mesoblast makes a border 
on each side of the spinal cord extending but a short distance laterally 
over the yolk. The anterior three-fifths of it is divided into ten somites 
opposite the third and fourth of which it is thickened forming the begin- 
ning of the anterior limb. ‘The posterior two-fifths of the mesoblast is 
not yet divided into somites. The medullary folds flatten and a little 
in front of the well marked blastopore disappear. 
Larva 5.5mm. Figure 6. 
The pharyngeal region is now distinct and shows four gill arches; 
sections, however, show that the gill slits are not yet perforate. Pos- 
teriorly a crescent shaped furrow, the horns pointing forward, defines 
the termination of the body and marks its tendency to rise off the yolk. 
