SEGMENTATION OF OVUM 



25 



HISTORY OF THE SOMA. 



FORMATION OF THE BLASTODERM AND EMBRYONIC AXIS. 

 SEGMENTATION OF THE OVUM. 



Immediately after the sperm- and germ-nuclei have conjugated, the egg 

 divides into two segments or blastomeres (fig. 34). The division of the cell-body 

 is preceded, as in ordinary cell-division, by the mitotic cleavage of the nucleus, 

 and, as previously stated, each daughter- nucleus receives an equal complement 

 of paternal and maternal chromosomes. Each blastomere now divides to form 

 a group of four segments, which again cleave into eight, and the process of binary 

 division continues until a mass of small nucleated segments is formed, called the 

 mulberry mass or morula (fig. 34). This is enclosed by the zona radiata, and 

 is little, if at all, larger than the single ovum which it replaces within the zona. 

 The segmentation of the mammalian egg is complete or holoblastic (see p. 27), 



z.p. 



FIG. 34. FIRST STAGES OF SEGMENTATION OF A MAMMALIAN OVUM : SEMI-DIAGRAMMATIC. 

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



z.p., zona pellucida ; pgl., polar globules ; a, division into two segments ; Ic, larger and clearer 

 segment ; sc., smaller, more granular segment ; b, stage of four segments ; c, eight segments ; 

 d, e, succeeding stages of segmentation showing the more rapid division of the clearer segments 

 and the enclosure of the darker segments by them. 



but is neither quite equal nor quite regular. Some of the cells divide more rapidly 

 than others, so that groups with an odd number of segments occur, such as 3, 6, 12 

 or 7, 9, 10 ; and when the morula stage is reached there is a definite grouping of the 

 segments, the centre of the sphere being occupied by larger, more granular, cells, 

 surrounded by a layer of smaller, clearer elements. It is not certain at what stage 

 the distinction between the two categories of cells is established, though some 

 believe that it is effected at the first cleavage ; but it is clear (even though the 

 evidence for a definite epiholic process is incomplete) that certain of the cells divide 

 more rapidly, take a superficial position, and come to cover and enclose the 

 remainder. 



The cleavage of the human ovum has not been observed, and only one stage has so far been 

 seen in any of the lower Primates (fig. 35). It is a four-cell stage found in an oviduct of 

 Macacus nemestrinus given to Selenka by Hubrecht. ! The blastomeres are of nearly equal size, two 



1 Selenka, Studien iiber Entwickelungsgeschichte der Tiere, Heft x., Wiesbaden; Kreidel, 1903, 

 p. 831. 



