732 LIFE : OUTLINES OF GENERAL BIOLOGY 



and centrosome. In artificial parthenogenesis the normal spermic 

 stimulus is replaced by some other, usually chemical or physical, 

 and, as the resulting development is normal, it follows that the 

 ovum-nucleus must be complete in itself, even in cases where parthe- 

 nogenesis does not occur in natural conditions. In the threadworm, 

 Rhabditis, the spermatozoon acts as a stimulus, yet degenerates 

 without fusing with the egg-nucleus, an interesting separation of 

 the two functions of amphimixis and stimulation. In echinoderms it 

 has been shown that if the fertilised ovum is cut in two before the 

 sperm-nucleus has united with the egg-nucleus, the half containing 

 the former may divide and develop, while the half containing only 

 the latter degenerates and dies! It is interesting to compare this 

 case of "Merogony" (development of apart) with what happens in 

 the Rhabditis ovum. For what develops in the former is a piece of 

 ovum with only a sperm-nucleus, while what develops in the case 

 of Rhabditis is an ovum with only an ovum-nucleus. 



SEGMENTATION. — The pole of the ovum towards which the 

 nucleus or the pure formative protoplasm is nearest is called the 

 animal pole, and it is there that the polar bodies are given off. It is 

 the region of greatest chemical activity, greatest permeability, 

 greatest susceptibility to environmental changes; and it is electri- 

 cally negative to the rest of the protoplasm. The opposite pole, 

 where there is most yolk-material, is called the vegetative pole, 

 and in a freely floating egg it is usually lowest, as is readily seen 

 in a frog's egg. In the floating ova of some fishes, however, the yolk 

 is uppermost (often showing a buoyant oil-globule), and the embry- 

 onic area is lowest. 



After fertilisation, but before the division of the egg into the two 

 first blastomeres, there may be a visible arrangement of the 

 materials in the cytoplasm ; and the cleavage may follow so regular 

 a pattern that it is possible to say that a particular spot in the 

 ovum will have a certain role in development. Some ova are more 

 visibly mapped out than others, but the mapping out is not radically 

 important, since a complete embryo may be developed from a portion 

 of an ovum, or from one of the first two blastomeres. Moreover, a 

 complete disturbance of the visible pattern of the egg by whirling 

 in a centrifuge may not involve any abnormality in the development. 



The plane of the first cleavage of the fertilised egg-cell is typically 

 a meridian running through the two poles of the egg, and its exact 

 situation is determined by the path of the sperm's nucleus and 

 centrosome through the cytoplasm. In simple cases, well illustrated 

 by the frog's egg, the first cleavage divides the egg into prospective 

 right and left halves; the second, at right angles to the first, into 

 prospective anterior and posterior halves. Then follows an equatorial 

 cleavage, at right angles to the two preceding, and this divides the 



