16 



SEGMENTATION OF THE OVUM. 



EARLY CHANGES IN THE OVUM CONSEQUENT ON FERTILIZATION SEGMENTATION 

 FORMATION OF THE BLASTODERM THE PRIMITIVE STREAK AND GROOVE. 



Segmentation of the ovum. Immediately after the completion of the process 

 of fertilization, the ovum begins to show signs of division into two cells or segments. 

 The division is preceded by the formation of a spindle-shaped system of achromatic 



Fig. 13. FIRST STAGES OP SEGMENTATION OP A MAMMALIAN OVUM : SEMI-DIAGRAMMATIC. 

 (Drawn by Allen Thomson after E. v. Beneden's description.) 



z.p, zona pellueida ; p.gl, polar globules ; a, division into two segments ; ect, larger and clearer 

 segment ; ent, smaller, more granular segment ; b, stage of four segments ; c, eight segments, the ecto- 

 meres partially enclosing the entomeres ; d.e, succeeding stages of segmentation showing the more rapid 

 division of the clearer segments and the enclosure of the darker segments by them. 



fibres and by changes in the nucleus which are similar to those which take place in 

 the division of an ordinary cell (v. Histology). According to v. Beneden's observa- 

 tions in Ascaris these changes occur in each of the two pro-nuclei (fig. 12), and 

 one-half the number of resulting V-shaped filaments then passes from each to form 

 each daughter nucleus, which thus contains male and female chromatin elements in 

 equal amount. Each of the two segments which are thus formed speedily again divides 

 in the same manner, so that four cells or segments now occupy the interior of the ovum. 

 By a further process of binary division eight cells are formed, then sixteen, thirty-two, 

 and so on until the originally simple ovum is eventually subdivided into a large 

 number of small segments, each of which is a nucleated cell, which are aggregated 

 into a solid spherical mass, not much larger than the original ovum, and known as 

 the mulberry mass. The cells are not similar throughout, for those at the surface 

 are clearer and less granular than those which occupy the interior of the mass. 

 According to v. Beneden's observations in the rabbit and bat, this difference in the 

 appearance of the cells is traceable even in the first pair of daughter cells, one of 

 which is larger and clearer than the other, which is darker and more granular (fig. 

 13, a). 1 The cells or segments which result from the division of each of these retain 

 their respective characters, and since the clearer cells divide somewhat more rapidly 

 than the darker ones, there are for a time at certain stages of the process of segmen- 

 tation more of the clear cells ; thus, at one stage there are eight clear cells, and only 

 four darker ones, the latter having not yet undergone division, and later there are 



1 This statement is denied by Kblliker and other observers, who assert that there is no difference in 

 the size and appearance of the first segments. 



