. A manual of zoology. Zoology. GENERAL EMBRYOLOGY. 149 of fertilization, for even liere but one spermatozoon fuses with tlie egg- nucleus, the others degenerating sooner or later. Essential Feature of Fertilization.—After the spermatozoon has penetrated into the egg, the head and the middle piece which contains the centrosome can still he recognized, as the chromatic and acliromatic parts of the si^ermatozoou or sperm-nncleus (male pronucleus), while the tail and the slight amount of protoplasm disappear in the yolk. In the cytoplasm of tlie egg the centrosome of the sperm-nucleus gives rise
. A manual of zoology. Zoology. GENERAL EMBRYOLOGY. 149 of fertilization, for even liere but one spermatozoon fuses with tlie egg- nucleus, the others degenerating sooner or later. Essential Feature of Fertilization.—After the spermatozoon has penetrated into the egg, the head and the middle piece which contains the centrosome can still he recognized, as the chromatic and acliromatic parts of the si^ermatozoou or sperm-nncleus (male pronucleus), while the tail and the slight amount of protoplasm disappear in the yolk. In the cytoplasm of tlie egg the centrosome of the sperm-nucleus gives rise to conspicuous rays, like those observed during division. Preceded by these rays the sperm- nucleus travels towards the egg-nucleus until it reaches (fig. 9-t); and fuses with it to form a single cleavage nucleus. Now the centrosome divides into two, which migrate to opjjosite poles of the nucleus, while the cleavage nucleus changes to a cleavage spindle, which divides and thus initiates the embryonic develop- ment, the successive divisions being known as the cleavage or seg- mentation of the egg. iSince not until this point is fertilization complete, we arrive at the fundamentally important proposition that the essentuti feature of fertilization consists in the union of eqq unit sperm Fig. 04.—Stages in the fertilization of the egg of the sea-urriiin. (Aft(.'r O. Hei-twig.) The sperm-nucleus {sh) -n'itli its rays is near the surface in one egg, in the other near the egg-nuoleus (i7i). Part Played by the Two Nuclei in Fertilization.—In many cases an abbreviation of development may take place, the stage of the cleavage nucleus being omitted, and the egg and sperm nuclei, without jjrevionsly uniting, pass directly into the cleavage sjiindle. This fact in no wise alters the above-mentioned proposition, but yet it is important, because it shows more plainly in what way the two nuclei jiarticipate in the formation of the cleavage spindle. It shows that of the chromoso
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