Studies on the Mechanism of Cell Division 3, Penetration of Spermatozoon into Egg Interior,and Fusion of Both Pronuclei

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  • Studies on the Mechanism of Cell Divisi

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The unfertilized eggs of a sea urchin, Hemicentrotus pulcherrimus, were first immersed in hypotonic sea water, then, centrifugalized by an ordinary driven electric centrifuger. The eggs were remarkably stratified forming conspicuous clear hyaloplasmic layer. In the case of the sperm attachment at the site of this layer, the behavior of the spermatozoon in the process of invasion, and the course of penetration through the egg surface, were precisely observed. To ascertain the mechanism of synkaryon (fusion of two pronuclei), the fertilized eggs in the various stages of development were exposed to sea water containing phenylurethan. In the exposure, the formation of the astral rays and spindle fibers were completely suppressed. Through the experiments, the behavior of two pronuclei in each period of the exposure, and the recovering process were observed. As the results, following conclusions were obtained. Phenylurethan completely inhibits the formation of the astral rays which are radiating from the centrosome of the sperm head (the sperm head later swells into male pronucleus). Under these situations, the fusion of both pronuclei never took place as far as the writer's experiments were concerned. From the above facts, it seems clear, that the astral rays of the sperm pronucleus play an important role in the mechanism of the compromise and the fusion of two pronuclei. In the eggs, exposing to phenylurethan beginning ten minutes after insemination, and transferred to sea water within 45 minutes, synkaryon took place about 30 minutes later. In this case, the mitotic apparatus was rapidly formed. Cleavage began almost at the same time in the control eggs. The rapid formation of the mitotic apparatus and synchronous cleavage of the recovering eggs are probably based upon the adaptation to the endoplasmic condition of the treated eggs, advanced regularly irrespective of the immersion of the reagent.

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