486 PROCEEDINGS OF SECTION D. 
the line of the median keel at any stage, and figures them as 
clearly distinguishable from one another. 
It is some time before the blastoderm has quite covered the 
yolk that the earliest rudiments of the optic vesicles become 
visible. First the anterior end of the epiblast layer of the keel, 
which may be termed medullary cords, shows a rounded enlarge- 
ment in front. Then in this there become distinguishable an 
axial portion, which is the anterior end of the medullary cord, 
and two lateral parts, which soon become distinctly separated 
from the former. When they first become distinguishable these 
lateral parts of the anterior enlargement extend to the extreme 
front end of the latter; but very soon they appear to retreat 
backwards—their anterior ends falling short, by a little distance, 
of the end of the axial cord. This appears to be due to the 
anterior part thinning out, while the posterior part becomes 
thickened and more strongly defined. These lateral parts of the 
anterior enlargement are the equivalents of what Goette* terms 
the sensory plate (Sinnesplatte). The thickened posterior part 
forms the optic “ vesicle.” 
That these optic rudiments arise from the same stratum of cells 
as the medullary cord is evident enough, but from the way in 
which they make their appearance at the sides they would rather 
seem to be thickenings of the surface stratum of epiblast than 
outgrowths from the medullary cord. The latter, it has also to 
be noted, at the time when the bodies in question are distinctly 
formed is scarcely yet a definite structure, but is really nothing 
more than the more superficial cells of the blastodermic ridge or 
keel, which are not yet definitely marked off from those below, 
destined to form the notochord and the mesoblast. 
Up to this point there is not the least appearance of a groove 
or cleft on the surface of the medullary cord, which is an un- 
divided axial thickening of the epiblast not projecting very 
prominently on the surface. It is only after the establishment of 
the optic “vesicles” that the medullary cord becomes separated 
from the cells lying below it, and becomes marked out by a faint 
longitudinal fissure into two lateral halves. A little later a pair 
of thickenings appear behind the optic rudiments at the sides of 
that part of the embryonic cerebro-spinal axis destined to form 
the hind-brain. These when first they become evident are 
elongate thickenings running parallel with the medullary cord. 
The middle part of each gives rise to the rudiment of the auditory 
labyrinth, which makes its appearance later in the form of a 
rounded sac. 
The first trace of the drain is the appearance of a slight 
swelling just behind the optic vesicles. From this there become 
constricted off behind a pair of inconspicuous swellings, which 
* “ Beitriige zur Entwickelungsgeschichte der Wirbelthiece, IV., Ueber die Sinnesplatte 
der Teleostier.” Arch. f. Miker. Anat. 
