110 OEGANIZAIION AND DEVELOPMENT OF ANIMALS IN GENERAL. 



of a neiv element bringing about the regeneration of the primary 

 nucleus of the ovum or germinal vesicle, and would have impressed 

 its influence on the constitution of the conjugated nucleus. The 

 regenerated ovum is therefore the starting-point of the subsequent 

 generations of cells which build up the embryonic body. 



Both the origin of the polar bodies which takes place in the ripe 

 ovum independently of fertilization, and the division of the segmen- 

 tation nucleus are accompanied by the appearance of the nuclear 

 spindle and star- shaped figures at the poles of the spindle which are 

 so characteristic of the division of nuclei. The male pronucleus, 

 before it fuses with the female pronucleus, also becomes surrounded 

 by a layer of clear protoplasm, around which a star-shaped figure 

 appears (fig. 101). In those cases in which segmentation takes 

 place without a precedent fertilization (parthenogenesis], the female 

 pronucleus appears to possess within itself the properties of the first 

 segmentation nucleus. 



The fertilization is followed by the process known as segmentation, 

 in which the ovum gradually divides into a greater and greater 

 number of smaller cells. Segmentation may be total, i.e., the whole 

 ovum segments (fig. 103), or it may be partial, in which case only a 

 portion segments (fig. 105). 



Total segmentation may be regular and equal, the resulting seg- 

 ments being of equal size (fig. 103) ; or it may sooner or later become 

 irregular, the resulting segments being of two kinds the one smaller 

 and containing a preponderating amount of protoplasm, the other 

 larger and containing more fatty matter. In these cases the seg- 

 mentation is said to be unequal. The process of division proceeds 

 much more quickly in the smaller segments, while in the larger and 

 more fatty segments it is much slower, and may eventually come to 

 a complete standstill. The development of the frog's egg will serve 

 as an example of unequal segmentation, of which there are various 

 degrees (fig. 104). In this egg a dark pigmented and protoplasmic 

 portion can be distingirVied from a lighter portion containing 

 much fatty matter or food yolk. The former is always turned 

 uppermost in the water, and is therefore called the upper pole of 

 the egg. The axis which connects the upper pole with the lower 

 is known as the chief axis. The planes of the two first segmentation 

 furrows pass through the chief axis and are at right angles to each 

 other. They divide the egg into four equal parts. The third 

 furrow (fig. 104, 4) is equatorial, taking place in a horizontal plane, 

 and cuttin^ the chief axis at right angles. It lies, however, nearer 



