THE DEVELOPMENT OE SYMBRANCHU8 MARMORATUS. 
o 
Celloidiii sections through eggs of this stage show that this 
pad is more or less bi-coiivex in shape — 9 to 10 cells deep 
ill the centre. Already an outer layer of cells is becoming 
marked olf from the general mass, i . e . an ectoderm differen- 
tiating. Three distinct zones can be seen in the subjacent 
yolk, from which the pad can easily be separated. Under the 
central deepest part of the blastoderm the yolk is finely 
divided, while the lateral parts of the pad overlie a mass 
of protoplasm studded with large nuclei. Here the outer rim 
of the blastoderm shows more obvious continuity with the 
rest of the egg than it does over the central portions. This 
protoplasmic mass no doubt functions in connection with the 
passing on of food material to the quickly dividing cells in 
the blastodermic area. 
Protoplasmic masses and disintegrated yolk pass more or 
less suddenly into the large structureless yolk-blocks which 
constitute the vegetative part of the egg. A thin film of 
protoplasm acts as a covering to the yolk-mass, which is not 
yet covered by blastoderm. This covering is, however, a very 
slender protection, especially at the vegetative pole, where 
masses of yolk break away very readily when once the cap- 
sule has been removed. 
Stage 7. — As the blastoderm grows the central portion 
appears to thin out, while the rim is thickened and less white 
m appearance. The yolk nearest the blastoderm is much 
lighter in colour than the rest, which is brownish-yellow — 
this difference m colour being the external expression of the 
yolk differentiations described above. The celloidin sections 
show an ectoderm, now two-layered, and an irregular and very 
small segmentation cavity. 
Stage 10. — When the blastoderm has covered up about 
100° to 120° the thickening of the rim becomes accentuated 
at one point to form the embryonic rudiment (PL 1, fig. 2, 
and Text-fig. 1). In the paraffin sections (PI. d, fig. 17) there 
IS a space underlying the embryo rudiment which may be 
artificial or which may represent remnants of an archenteric 
cavity. Celloidin sections (PI. 3, fig. 18) do not show any such 
