730 LIFE : OUTLINES OF GENERAL BIOLOGY 



may become (as in sea-urchins) detached from the surface of the 

 ovum to form the "fertihsation membrane", the space between it 

 and the egg being filled with a clear fluid. This is a critical moment 

 in the fertilisation process, for there is a marked increase in the 

 permeability of the egg-surface, so that dissolved substance may 

 diffuse out from the cytoplasm, and also an increase in suscepti- 

 bility to external changes. Very rapidly the egg-cell is reawakened ; 

 there is a great increase in the metabolism after the fertilisation has 

 occurred — increased consumption of oxygen, increased evolution of 

 carbon dioxide, and increased production of heat! But a chemical 

 and physical change around the periphery of the ovum renders it 

 non-receptive to other spermatozoa. This "blocking" of the egg is 

 probably advantageous, for it has been observed that the entrance 

 of several sperms (polyspermy) is apt to set up several centres of 

 segmentation, the result being abnormality. Polyspermy is said to 

 be frequent among insects and cartilaginous fishes, but in these 

 cases only one of the sperms actually combines with the nucleus of 

 the ovum; the others come to nothing. 



In entering the egg-cell — it may be by an attracting hillock — the 

 spermatozoon leaves its locomotor tail outside; but in many cases 

 the middle piece seems to enter along with the head, and this may 

 introduce the centrosome. This proceeds to divide into two, which 

 play an important role in the segmentation of the fertilised ovum. 

 The immature ovum has a centrosome, but for some unknown 

 reason this disappears. It follows that the centrosome which 

 appears after fertilisation has either been introduced by the sperma- 

 tozoon, or evoked de novo by its influence on the protoplasm of the 

 egg. Some cytologists hold to the older view, that the spermatozoon 

 introduces a centrosome, which divides into two; and this is sup- 

 ported by the fact that in ordinary cell-divisions, centrosomes 

 usually arise from centrosomes. But other experts maintain that 

 the centrosome of the fertilised ovum is a new product, and this is 

 supported by a striking experiment made by Chambers, who, in one 

 of his masterly manipulations, cut ofi the whole of the entering 

 spermatozoon behind the head, and then observed the appearance 

 of a centrosome as usual in the fertilised ovum. It is possible that 

 the centrosome is introduced in some types and evoked in others. 



After movement through the cytoplasm of the egg, the nucleus 

 of the spermatozoon finds the nucleus of the ovum, and the result 

 of their union is the "segmentation-nucleus". The union is intimate 

 and orderly, but the reduced chromosomes of the sperm do not 

 fuse with the reduced chromosomes of the egg. They form pairs, 

 and each member of a pair is longitudinally split in all the subse- 

 quent divisions that are involved in the development of the embryo 

 and eventually the whole body of the offspring. In some cases, such 

 as the water-flea Cyclops, it has been possible to follow the early 



