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ignorance in which Ave were involved before this research was 

 undertaken. 



As in all biological investigations, it was necessary first of all 

 to find an organism or a cell which displayed in the most advan- 

 tageous manner the phenomena it was desired to investigate. For 

 this purpose such a cell was found in the egg of the sea-urchin — 

 a familiar organism, the shells of which are often observed on the 

 beach after a storm, and are colloquially named sea-eggs. The 

 egg of the sea-urchin presents many advantages. In the first 

 place it has a comparatively large cell. It may readily be seen, 

 and appears of a large size under the low powers of a microscope 

 and all of the mechanical phenomena occurring in it may easily 

 be observed and followed by the eye with such means. 

 The spherical cell has a very small nucleus which can hardly be 

 detected except by special methods of staining and examination. 

 These eggs are produced by a single female sea-urchin in millions y 

 and it is therefore possible to perform no less than a million 

 simultaneous experiments by simply taking the eggs from one 

 female. 



The spermatozoon, or male element of the sea-urchin, is a 

 very different type of cell. It is extremely small, and almost 

 entirely consists of nucleus. The normal fertilization of the sea 

 urchin egg takes place in the following manner. The eggs are 

 discharged into the sea by the female, and the spermatozoa are 

 also discharged into the sea. The two meet by accident. Only a 

 very small proportion of the eggs become fertilized, and from the 

 fertilized eggs a sea-urchin embyro results. From the embyro 

 which is a free-swimming organism — swimming by means of cilia, 

 or little hairs on the exterior surface, which propel the animal 

 through the water — the complete development into a fully grown 

 sea urchin is a matter of some two years. You will, therefore, 

 readily imagine that it is quite possible to fertilize the eggs 

 normally in the laboratory in a glass container, and watch the 

 processes going on. You only have to take some eggs out of the 

 female, and some spermatozoa from the male, and mix the two 

 together in sea water. We then observe the following phenomena. 

 The spermatozoa will be seen crowding around the egg. The 

 spermatozoa can swim freely by means of little hairs which they 

 possess at their posterior extremity. Hundreds of these will be 

 seen surrounding one single egg. One spermatozoon enters the 

 egg. In doing so it seems to cleave a sort of passage through the 

 egg, leaving a distinct trail behind. Almost the instant that it 

 enters the egg a peculiar appearance is observed. The egg itself 

 is rather granular and opaque, but immediately after fertilization 

 a small clear area begins to appear on the outside, which gradually 

 increases in width until we have the appearance of a granular 

 sphere enveloped in a clear transparent shell. This clear area is 

 surrounded by a membrane which pushes out the spermatozoa and 

 prevents any more from entering, so that, as a general rule and 

 under normal conditions, only one spermatozoon enters the egg. 

 The instant the spermatozoon enters the egg the formation of the 

 clear membrane prevents the entrance of any more spermatozoa. 



The events which succeed this are, so far as physical mani- 

 festation goes, very obscure and difficult to follow by the eye, but 

 of one event we may be perfectly certain — that the nucleus of the 

 spermatozoon and of the egg come together and fuse into one nu- 

 cleus. A point which it is rather important to recollect is that the 

 spermatozoon consists almost entirely of nucleus. Therefore 



