180 ADAM SEDGWICK. 
and placed immediately beneath the egg-shell around a central 
cavity—the segmentation cavity. Each ectoderm cell 
presents in the fresh specimen (Part 1, fig. 7) (1) a central 
clear area—the expression of the nucleus; (2) around this a 
dark area—the expression of the dense protoplasmic reticulum 
around the nucleus ; and (3) a paler circumferential area, which 
is more marked on the outer than on the inner border of the 
cells. This is the expression of the looser part of the reticulum, 
which is continuous internally with the reticulum of the 
adjoining cells, and externally with the clearer masses consti- 
tuting the rest of the ovum, and called by me the endoderm 
masses (Pl. XIII, fig. 19). All the above elements are 
arranged round the central cavity, which was present even in 
the unsegmented ovum. Fig. 14, Pl. XIII, is a diagram- 
matic representation of a transverse section through the ecto- 
derm cells at this stage; it shows the continuity of the looser 
circumferential parts of the reticulum of the two cells (the 
endoderm masses are not represented in this figure). 
The next divisions take place parallel to the long axis of the 
ovum, and result in the formation of sixteen ectoderm cells 
arranged in four rows, each row containing four cells. A 
diagrammatic transverse section of such an ovum is shown in 
fig. 15, Pl. XIII, in which the endoderm masses are repre- 
sented. This section also shows the segmentation cavity 
around which the various elements are arranged. 
The further changes which may be considered as belonging 
to the segmentation stages consist in the continued and regular 
subdivision of the ectoderm cells, and in the continued breaking 
up of the endoderm masses into smaller bodies, Fig 8 in 
Part 1 represents a fully segmented ovum. It consists of a 
small patch of ectoderm cells, and a number of irregular 
branched endoderm masses. Both the ectoderm cells and 
endoderm masses are placed immediately beneath the egg- 
membrane round the segmentation cavity. A diagrammatic 
representation of a transverse section of such an ovum is shown 
in Pl. XII, fig. 10, and Pl. XIII, fig. 17 is a drawing of an 
actual section through such an ovum in sittin the uterus. 
