48 PRINCIPLES OF EMBRYOLOGY 



impact of the first spermatozoan causes a cortical change in the egg which 

 spreads over the whole surface in a much shorter time than it takes for the 

 fertilisation membrane to appear. Rothschild and Swann (1949) were able 

 to reveal this change by the use of dark ground illumination, and they 

 present some reasons for thinking that it is this almost immediate effect, 

 rather than the relatively slow elevation of the fertilisation membrane, 

 which constitutes the block to polyspermy. 



A surface change which guards the egg from the entry of more than 

 one sperm can, however, occur without any visible sign of a fertilisation 

 membrane. In fact, a change of this kind seems to be a quite general part 

 of the activation process, excepting only in some of the very large, 

 extremely yolky eggs, such as those of reptiles and birds and some insects. 

 In these, the entry of considerably more than one sperm is a normal 

 occurrence; only one sperm nucleus fuses with the egg nucleus, and the 

 remainder gradually disappear after remaining for a time in the region 

 where the cytoplasm mingles with the yolk, in the digestion and assimila- 

 tion of which they may play a part (and see p. 62). 



Changes of the egg surface are not always the only visible signs of 

 activation. In many eggs, the penetration of the sperm initiates a more or 

 less complete rearrangement of the internal constituents (Fig. 3.3). Other 

 examples are described in detail later (see ascidians, p. 106, Amphibia p. 

 146). In these cases the pattern of the egg before activation bears Httle ob- 

 vious relation to that of the embryo which will develop from it, while 

 after activation a clear connection can be traced; thus we may say that in 

 these forms, activation is the final stage in preparing the egg for the series of 

 foldings and bendings by which the embryonic body will be shaped. 

 These internal results of activation can only be discovered at all easily if 

 there are differences in colour or texture between the various regions of 

 the egg which make it possible to follow their movements after the sperm 

 penetrates. It is therefore only in certain favourable types of eggs that 

 they have been described, and it is still somewhat uncertain how generally 

 they occur. The evidence suggests that there may always be some internal 

 rearrangement, but that it is often quite small in extent. 



One aspect of activation which may be specially mentioned is the deter- 

 mination of the plane of bilateral symmetry. Most eggs, as has been stated, 

 have before fertilisation an axis of symmetry running from the animal 

 pole (where the polar bodies are formed) to the vegetative (yolky) pole. 

 Some eggs (e.g. of insects) are bilaterally symmetrical, but in most types 

 there is no sign in the unfertilised egg of anything corresponding to a 

 'Greenwich meridian' ; and it may turn out in later development that the 

 plane of bilateral symmetry is related to the point of entry of the sperm. 



