366 
PROFESSOR E. RAY LANKESTER. 
cells/^ are very delicate, often much stretched and translucent, 
but in order, as it were, to fill up the interstices of the spherical 
mass, they are sometimes enlarged in places and refringent like 
the medullary cells. 
The cortical cells are, however, never so distinctively modified 
as the medullary cells. There is a peculiar brownish colour and a 
refringent appearance about the medullary cells, especially after 
treatment with osmic acid, which is not to be noticed in the 
cortical cells. 
The medullary cells sometimes contain one or two greenish- 
yellow granules (figs. 8, and 15 gr\ which are identical in ap- 
pearance with the granules found in the endoderm cells of the 
marginal ring canal/ro?^ which they are derived. 
By searching along the velo-marginal line it is easy in some 
specimens to find refringent bodies and their capsules in all 
stages of development. The earliest indication which I have 
obtained of this development is a protrusion of the endodermic 
cells of the marginal canal into the ring or band (^) of colourless 
cells, which is ectodermal in origin, and probably to a large 
extent nervous in character. This protrusion (PI. XXXI, figs. 12, 
13), lifts the ectodermal tissue in front of it, and one of the in- 
growing endodermal cells enlarges and becomes highly charged 
with fine granules, and sometimes with coarser, green-coloured 
granules also. As far as I have been able to ascertain, this enlarged 
endodermal cell, which frequently is coloured like the other endo- 
dermal cells of the ring- canal, is the mother-cell, from which the 
axial or medullary cells of the refringent body are developed (see 
PL XXX, figs. 14, 17, 18, axen). The ectodermal tissue into which 
this enlarged cell has protruded now grows around it in a very re- 
markable way. In some cases it seems to form at first a complete 
investment of small cells — the cortical layer — enclosing the en- 
dodermal medullary cell. But this appears from such a con- 
dition as that represented in PI. XXXI, fig. 14, not to be the 
actual mode of growth. The ectodermic layer, pushed forward by 
the axial outgrowth of endoderm, arranges itself in such a way as 
to form a cortical layer to the medullary cell, and at the same 
time a capsule embracing the central body formed by these two 
sets of cells (see PI. XXXI, figs. 15, 16). From henceforth the 
medullary cell has merely to divide and give rise to the four or 
eight highly refringent medullary cells of the fully-formed re- 
fringent body, and the capsule has only to expand and increase 
its area by the agglutination of vacuolated ectoderm cells in the 
manner described above. 
The ectodermic cortical investment of the highly refringent 
central or medullary cells of the marginal body is complete ; 
the endodermic medullary cells are entirely cut off from their 
