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ADAM SEDGWICK. 
which the embrvo floats. The membrane, therefore, has much 
the same function as the amnion of the Vertebrata. 
The protoplasm of the ovum 1 is differentiated into two parts — 
the main mass being of a pale colour with relatively few dark 
granules, while at one point it is especially dark in colour. This 
small dark patch (fig. 1) is placed at the surface on one of the 
long sides of the ovum. I may call it, from its position as 
determined by the later development, the dorsal or animal pole 
of the ovum. When the ovum is viewed from the side (fig. 1), 
it is seen that the surface of the dark patch is pitted inwards, 
and that the space so formed contains two small clear bodies, 
which I take to be polar bodies. When viewed from the face, 
the dark patch presents a central circular transparency more or 
less free from the dark granules which are found in such large 
numbers in other parts of it. This central clear body I take to 
be the first segmentation nucleus. The polar bodies are only 
seen during this stage, and I have no observation either on their 
origin or fate. 
I have figured two other unsegmented ova (figs. 2 and 3) 
which differ in certain respects from the above. In one of 
these (fig. 2) the dark patch is smaller than in fig. 1, and 
without the central transparent area ; in the other (fig. 3) there 
were several dark patches, each with its own clear spot. I have 
not been able to make out the meaning of these differences, 
i.e. whether they are normal stages in the development of the 
ovum, and if so, where they come in the developmental series. 
Segmentation. 
Segmentation is complete. The first furrow is in the trans- 
verse plane of the ovum, and divides it into two halves (fig 4), 
the dark patch being divided as well as the main mass of the 
ovum. The second furrow is at right angles to the first, and 
divides each of the first formed segments into two (figs. 5 and 6), 
so that the ovum now consists of four segments, each con- 
sisting of a lighter-coloured main mass and a small dark patch 
1 The following description of the segmenting ova refers, unless otherwise 
stated, to fresh living ova seen in transmitted light. 
