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until we find evidence to the contrary, we may consider that the 

 nucleus of the spermatozoon is the most important part of it. In 

 spite of this fact, however, it has been proved that a fusion of 

 these two nuclei is not absolutely necessary for fertilization, 

 although it occurs normally. It is possible, as shown by Boveri, 

 to cut a piece of the spherical egg, not containing any of the 

 nucleus, and to fertilize that by means of spermatozoa, and to 

 obtain an embryo. 



Although the visible mechanical phenomena which succeed 

 fertilization are very difficult to follow, the chemical phenomena, 

 thanks to the investigation of Loeb, are now rather well under- 

 stood. The chemical process which unquestionably follows the 

 fertilization of an egg is primarily an oxidation — a combustion, 

 or burning up of some material. In all combustions oxygen is 

 used up — oxygen derived from the air — and oxygen is used up in 

 the development of the sea-urchin's egg. Normally, after the stage 

 which I have described has been reached, after a pause of greater 

 or less length the egg within the clear surrounding membrane 

 divides into two. The nucleus also divides into two, and we have 

 two nuclei. Following this, each cell divides into two, and this 

 process of division continues until we have a sort of mulberry- 

 shaped object lying within this clear membrane. As development 

 proceeds, thousands of cells are produced by this single original 

 cell until a sort of sac is produced, and finally this sac breaks 

 through the clear membrane, and develops into a pyramid-shaped 

 embryo, with a perfectly defined skeleton in it, and with swim- 

 ming cilia on the outside. All this can be readily followed with 

 the eye under the microscope, but the whole process depends on 

 the presence of oxygen in the waters in which it occurs. If we 

 deprive the original cell of oxygen, by no means whatever can 

 this development occur. We do not kill the cell by depriving it of 

 oxygen, because after having done so for many hours, and com- 

 pletely arresting development thereby, on letting in oxygen again 

 development goes ahead just as if it were normal. 



Now, you recollect I told you that not only does the original cell 

 divide into two after fertilization, but also the nucleus, and the 

 strange pari} of it is that at the end of the division each of the 

 nuclei is as large as the original one. Moreover, when the original 

 cell has divided into four, each of the nuclei is as large as the 

 original single nucleus. The same is true when we have 64 or 

 any number of cells. The total number of cells, however, is not 

 much larger than the original cell, because it all takes place 

 within the membrane, which prevents the embryo from increasing 

 in size in the early stages, but the quantity of nucleus material 

 within the membrane is increased enormously. The material of 

 the nucleus of the cell differs from most of the material occurring 

 elsewhere in the cell by its very high content of phosphorus, and 

 the question arises where does the phosphorus contained in these 

 nuclei come from? In older days it was generally assumed that 

 it came from the sea water, which does contain a little phosphorus 

 in the form of phosphoric acid, but it is possible to carry out the 

 entire process in artificial sea water made up in the laboratory 

 which contains no phosphorus whatever, and the development 

 occurs precisely as it does in sea water taken from the ocean. 

 Therefore the phosphorus certainly does not come from the sea 

 water. There is only one substance within the cell from which 

 all that phosphorus can be derived, and that is a substance 

 of a fatty nature known as lecithin. It can be very easily shown 



