204 GENERAL PHYSIOLOGY 



and become surrounded each with its own protoplasmic radiation. 

 Thus the fertilisation is ended and the division of the fertilised 

 ovum into the first two cleavage-cells is introduced. But, unfor- 

 tunately, this account by Fol concerning the course of fertilisation 

 and the much-quoted " quadrille of the centrosomes " appears to 

 rest upon incorrect observation. At least Boveri ('95) and, in 

 harmony with him, Wilson and Mathews ('95) in sea-urchin 

 eggs, and Mead ('95) in the eggs of tube-worms (Chcetopterus 

 pergamentaceus), have found that such a quadrille of the centrosomes 

 does not exist, that rather the centrosome of the egg-cell perishes 

 and disappears (Mead) without playing any rdle, while that of the 

 sperm-cell after fertilisation divides alone in the egg-cell into two 

 centrosomes, each of which becomes a centre for the protoplasmic 

 radiation and the succeeding division of the fertilised ovum. 



A re'sume' of the essential factors of the phenomena of fertilisa- 

 tion leads to the following statement : Fertilisation consists in the 

 union of two cells, the egg-cell and the sperm-cell, in which protoplasm 

 fuses ivith protoplasm and nucleus with nucleus ; thus, in the succeed- 

 ing division of the fertilised egg-cell each half obtains material from 

 both the fused cells, and from both the protoplasm and the nucleus. 



4. The Development of the Multicellular Organism 



Development may be defined in a general sense as a continuous 

 series of changes. If we leave out of consideration the repro- 

 duction of the multicellular organism by the constriction of 

 entire parts of the body, as in gemmation and fission, where the 

 essential cell-groups of the individual systems of organs are 

 transferred directly from the parent organism to the buds or 

 products of fission, the formation of the multicellular organism 

 consists only in its development from the egg-cell. The multi- 

 cellular organism develops gradually from a single cell, whether 

 the egg develops without fertilisation, as in the interesting- 

 phenomenon of parthenogenesis (which occurs in certain lower 

 animals and affords a real background for the ancient legend of 

 the immaculate conception), or whether the egg has previously 

 been fertilised, as is the general rule in the development of 

 animals and plants. 



Development is present in unicellular organisms, but here 

 the whole cycle proceeds in a single cell. The development 

 of the Protista forms an interesting analogy to that of multi- 

 cellular organisms, both animals and plants. In the lowest forms, 

 such as Amceba, development is identical with simple growth. An 

 Amoeba changes simply by increasing in mass and then dividing. 

 The halves then grow again until they become so large that they 

 again divide. The whole developmental cycle of Amceba consists 

 in growth up to cell-division. We see, therefore, that growth and 



