CHAPTER III. 



THE SEGMENTATION OF THE OVUM. 



THE immediate result of the fusion of the male and female pro- 

 nucleus is the segmentation or division of the ovum usually into 

 two, four, eight, etc. successive parts. The segmentation may 

 be dealt with from two points of view, viz. (i) the nature of the 

 vital phenomena which take place in the ovum during its 

 occurrence, which may be described as the internal phenomena 

 of segmentation. (2) The external characters of the segmenta- 

 tion. 



Internal Phenomena of Segmentation. 



Numerous descriptions have been given during the last few 

 years of the internal phenomena of segmentation. The most 

 recent contribution on this head is that of Fol (No. 87). He 

 appears to have been more successful than other observers in 

 obtaining a complete history of the changes which take place, 

 and it will therefore be convenient to take as type the ovum of 

 Toxopneustes (Echinus) lividiis, on which he made his most 

 complete series of observations. The changes which take place 

 may be divided into a series of stages. The ovum immediately 

 after the fusion of the male and female pronucleus contains a 

 central segmentation nucleus. 



In the first stage a clear protoplasmic layer derived from the 

 plasma of the cell is formed round the nucleus, from which there 

 start outwards a number of radial striae, which are rendered 

 conspicuous by the radial arrangement of the yolk-granules 



