AMPHIMIXIS 49 



§ 7. Amphimixis and the Dual Nature of Inheritance in 

 Sexual Reproduction 



Apart from exceptional cases, the inheritance of a multi- 

 cellular animal or plant is dual — part of it comes from the mother 

 and part of it from the father ; in other words, the material 

 basis of inheritance is a fertilised egg-cell. The new individuality 

 has its origin in the fusion of two potential individuals, for as 

 such the ovum and spermatozoon must be regarded. The 

 exceptions referred to are cases of asexual multiplication by buds 

 or otherwise, as in the freshwater Hydra ; cases of partheno- 

 genesiSj as in the case of the unfertilised eggs which develop 



Fig. 12. — Fertilised ovum of Ascaris. (After Boveri.) 



clir. chromosomes, two from ovum-nucleus and two from sperm-nucleus ; cs. centrosome, 

 from which. " archoplasmic " threads radiate, partly to the chromosomes. 



into green flies (Aphides) in the summer ; and cases like liver- 

 flukes, where an animal is both mother and father to its offspring. 

 Apart from these exceptions the inheritance does at the start 

 consist of maternal and paternal contributions in intimate and 

 orderly union. 



When a spermatozoon, outstripping its fellows (for there are 

 usually very large numbers), reaches an ovum and bores its way 

 into it, the cytoplasmic flagellum is left behind, having performed 

 its function, and the sperm-nucleus and the ovum-nucleus move 

 towards one another. By a rapid change in the periphery of 

 the ovum, the enveloping membrane becomes firmer, and the 

 ovum becomes non-receptive to other spermatozoa. When 



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