THE DEVELOPIMENT OF SYMimANOHUS MARMORATUS. 7 
The small size ot‘ the egg and the coiTesponding small size 
and flattening ot’ the embryonic rndiment make it practically 
impossible at this stage to do more than distinguish the brain 
i*udiinenc from that of the neural tube and inyotomes. A 
faint median longitudinal groove can be seen externally. 
A microscopic examination of sections of the egg shows that 
the broad anterior part of the rudiment consists of a wide- 
spreading, solid medullai-y plate overlying a one-cell-deep 
plate of endoderrn, there being little mesoderm. In the 
middle parts of the rudiment a solid keel — the future spinal 
cord — is quite easily distinguishable from the mesoderm 
;md from the endoderm. There is no certain trace of noto- 
chord yet. 
The groove alluded to above is the external expression of 
a very shallow mid-dorsal depression found in the anterior 
parts of the embryonic rudiment, which disappears when 
the flat, wide-spreading medullary plate is converted into the 
solid uprising structure which forms the central nervous 
system just before the cavities are hollowed out. 
Stage 17. — When the still uncovered circular yolk-patch 
measures about 48° the embryo rudiment is about T8 to 2 mm. 
in length (PI. 1, fig. 4), though there are great variations, 
length being not always proportional either to differentiation of 
embryo or to size of uncovered yolk-patch. By this time the 
front part of the medullary ]3late has begun to rise from the 
egg, and to form the solid brain, and this uprising gradually 
extends backwards to the rest of the neural rudiment. The 
myotome rudiments are not yet differentiated into dorsal and 
ventral portions. The notochord is present, but its deriva- 
tion is difficult to observe. The solid rudiments of nose, eyes 
and ears are not yet distinct, though in the next stage — when 
the lumen of the nervous system appears — they are easily 
distinguishable in sections and their ectodermal connections 
can also be seen. Externally, howevei*, these organs are not 
yet visible. 
As development proceeds, liquefied yolk or oil, of which 
there is much in the yolk, seems to concentrate in front of 
