REPRODUCTION. 643 



In connexion with the development of the gametes, it will 

 be convenient to discuss the nature of the sexual process. 

 We already know that it consists in the fusion of the pro- 

 toplasm of two sexual reproductive organs, and that the 

 fusion may take place either within or outside them. But it 

 appears that the essential part of the process is the fusion of 

 the two nuclei. It has been observed in the case of lowly- 

 organised plants, such as Spirogyra and Pythium, that not 

 only does the protoplasm of the two gametes fuse into one 

 mass, but that the two nuclei do so likewise. This fact does 

 not, of course, prove that the fusion of the two nuclei is the 

 essential part of the process ; but the observation of the pro- 

 cess in plants which have well-differentiated gametes proves 

 that it is. The antherozoid of such plants consists, with the 

 exception of the cilia, almost entirely of nuclear substance : for 

 instance, as Strasburger points out, the antherozoid of Fucus 

 consists of a mass of nuclear substance enveloped by a delicate 

 layer of protoplasm which includes the "eye-spot," and is 

 prolonged into the cilia. Since it cannot be doubted that the 

 antherozoid fertilises the oosphere, it is clear that a quantity 

 of cell-protoplasm (cytoplasm) is not essential to the process. 

 This is even more strikingly brought out in the process of 

 fertilisation in Phanerogams, in which no protoplasm accom- 

 panies the nucleus (male pronucleus) derived from the pollen- 

 tube, as it enters the oosphere to fuse with the nucleus of the 

 oosphere {female pronucleus]. 



The details of the sexual process in Phanerogams are, according to 

 Strasburger, as follows. In the Gymnosperms a generative nucleus or 

 male pronucleus escapes through the mucilaginous end of the pollen-tube, 

 enters the oosphere, travels to the female pronucleus, and fuses with it 

 (see Fig. 71, p. 617, Fig. 76). In the Angiosperms, when the pollen-tube 

 comes into contact with the synergidae, a portion of its protoplasm enclos- 

 ing a generative nucleus passes out through the mucilaginous apex of 

 the pollen-tube, and travels between the disorganised synergidas to the 

 oosphere. The generative nucleus or male pronucleus then enters the 

 oosphere, leaving behind it the protoplasm which had served as a vehicle, 

 and fuses with the female pronucleus (Fig. 75). 



Before we enter upon the discussion of the physiological 

 significance of these facts of reproduction, we will turn our 



412 



