514 Embryonic History of the Lens in Bdellostoma 
Very early embryos of Bdellostoma in which the nose is still a single 
tube, and in which six or seven gill slits are present on the laterally 
outspread plates, will show the lens in the following condition: A small 
antero-dorsal portion of the irregularly shaped optic-cup comes in 
contact with the ectodermal head-wall, and from this ectoderm a pro- 
jection of cells extends inward toward the cavity of the optic-cup. The 
lens-bud is thus to an extent conical in form and results from a contact 
of only a portion of the optic-cup with the ectoderm. This structure 
continues to develop for a time until in an embryo considerably more 
advanced and measuring 15 mm. in length one sees the lens-bud with 
a shght indication of a constriction about the periphery of its area 
of union with the ectoderm, as if it were preparing to pinch off (Fig. 1). 
Here the progressive development of the lens ceases and degeneration 
begins. At this stage also the contact of the optic-cup with the ectoderm 
or lens-bud is just being lost. 
An older embryo in which all of the gill clefts have appeared, but 
are still on the outspread lateral plates, and in which the nose exists 
as two parallel tubes, shows the lens much reduced in extent. The optic- 
cup is now well separated from the ectodermal wall and a considerable 
layer of mesenchymous tissue is seen between the two (Fig. 2). The 
lens, L, here is indicated only by a slightly thicker area of ectoderm 
over the deeply buried optic-cup. In all embryos older than this one 
no indication whatever of a lens-like thickening could be found, the ecto- 
derm over the eye region being of the same thickness as that of adjacent 
areas (Fig. 3). This figure also shows that the optic-cup has continued 
to differentiate its parts, and is probably not so degenerate as to be 
unable to influence the ectoderm should it remain in contact with it. 
It is thus shown that the lens is not normally self-differentiating but 
begins to degenerate when contact with the optic-cup is lost. 
The embryos of Bdellostoma illustrate, therefore, by the changes 
which their lenses undergo many of the points sought in the above- 
mentioned experiments. They clearly show that the lens formation 1s 
directly dependent upon a contact of the optic-cup with the ectoderm. 
Secondly, contact with only a portion of the optic-cup is necessary to 
cause the ectoderm to begin lens formation. Thirdly, to produce a lens 
the contact of the optic-cup with the ectoderm must be durable; and 
fourthly, the optic-vesicle may change into an optic-cup without the 
aid of the mechanical pressure of the lens. This series of events would 
be difficult to interpret without the facts demonstrated by the experi- 
ments, while on the other hand it adds strength to the conclusions drawn 
