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 maybe dealt 

 with from two points of view, viz. (1) 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 segmentation. 



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 there- 

 fore be convenient to take as type the ovum of Toxopneustes (Echinus) 

 livid us, 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 centra! 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 between them. The 

 nucleus during this process remains perfectly passive. 



In the second stage the nucleus becomes less distinct and 

 somewhat elongated, and around it the protoplasmic layer of the 

 earlier stage is arranged in the form of a disc-shaped ring, com- 

 pared by Fol to Saturn's ring. The protoplasmic rays still take 

 their origin from the perinuclear protoplasm. This stage has a 

 considerable duration (20 minutes). 



