136 
Transactions of the Society. 
In figs. 19 and 20 are shown three such cells from late 
gastrulae. The smallest cell in fig. 20 has a small nucleolus just 
forming as the chromosomes are diffusing after cell division. In 
the other cell the nucleolus is larger relatively and is vacuolated. 
The larger mesodermal cell in fig. 20 contains a considerable 
amount of nucleolar substance. The granules NX very probably 
represent parts undergoing extrusion. Similar activities are 
represented in the cells marked ME in fig. 24. The condition of 
the nucleolus in the mesoderm cell shown in fig. 21 is unique. 
A part of it is prolonged into a kind of pseudopodium ; the greater 
part of it is spherical, and consists of a peripheral layer of 
markedly oxyphil substance (NR), internal to which is a less 
oxyphil staining area which appears to be a kind of vacuole. It 
seems probable that the substance marked NX represents a distal 
extruded portion of the pseudopodial process NP. 
The endodermal cells during development undergo considerable 
changes. At fig. 22, two such cells from a recently formed 
gastrula are shown. In both a large nucleolus is present in the 
nucleus, and within the cytoplasm are numerous vacuoles (V). 
In the upper cell the nucleolus is double, the larger part being 
vacuolated ; the nucleus on the right-hand side is somewhat 
concave, and within the concavity is the granule NX, surrounded 
by a clear region of the cytoplasm. Either this granule is a 
metaplastic one, and is the cause of the concavity of the nucleus, 
or it represents a portion of the nucleolus which has been extruded 
and is dissolving in the cytoplasm. From its size, staining 
reaction, and general disposition, I am inclined to favour the latter 
view as to its origin. 
The Nucleolus in the Larval and Adult Stages. 
From endodermal cells such as are shown in fig. 22, develop 
large cells such as the one drawn at fig. 23. The numerous small 
vacuoles increase in size and number and come together to form 
one or two very large ones, which lie at the end of the cell, distal 
to the archenteron. The nucleus also enlarges, and most charac- 
teristic of it, is the large nucleolus, which is usually vacuolated. 
Some of the cells in the region of the stomodseum of a larva 
are shown in fig. 24. An ectodermal cell at prophase is shown at 
CP. With the formation of the chromosomes (CH), the nucleolus 
has disappeared. Another ectodermal cell at telophase is shown 
at CT ; amongst one of the groups of chromosomes is seen a small 
red sphere which would appear to be the beginning of the 
nucleolus. All the cells in the " resting ” condition have a 
nucleolus, and in some of them are shown granules (NX) which 
are very possibly nucleolar in origin. 
Figs. 25 and 26 show cells from a later larval form — the free- 
