8 GENERAL DEVELOPMENT OF THE EMBRYO. 



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 on surface view during this stage, and I have no observa- 

 tion on their fate. 



I have figured two other unsegmented ova (figs. 2 and 3) 

 which difi'er 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. 



General Features of Segmentation. 



The segmentation is apparently complete ; i. e. the whole 

 ovum appears to break up into separate cells. But as a matter 

 of fact this is not the case ; the so-called segments do not, as 

 will be shown in the next chapter, completely separate from 

 one another. For the sake of convenience, however, the 

 terms " segmentation " and " furrow " will be employed in 

 the present general account. 



The first furrow is in the transverse 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 consisting of a 

 lighter-coloured main mass and a small dark patch which 

 closely adjoins the dark patches of the three other cells at the 

 animal pole, and which contains a central clear area (fig. 5). 



The two first furrows, therefore, are at right angles to one 

 another, and in the vertical plane. The next furrow is also 



