42 



PRINCIPLES OF EMBRYOLOGY 



It IS Within the egg-cell, forming an external layer known as the ecto- 

 plasm or cortex. It may be entirely clear, or it may contain a special 

 layer of granules; and it may have a definite external pelHcle or viteUine 

 membrane-m any case, we shall see that a new membrane frequently 

 forms from it at fertilisation. The cortex is so stiff that it is difficult to 

 move by centrifuging, and, in eggs where normal development occurs 

 after rearrangement of the interior, it seems that the reason is that the 

 essential normal structure is still retained by this elastic external layer 

 We know very little about the nature of this essential structure. It is often 

 spoken of as a 'gradient'; but this means little more than that the animal 

 end of the egg differs from the vegetative and that there is a gradual 

 transition between them. Probably the essential properties of the differ- 

 ent regions depend on sub-microscopic structures in the protein frame- 

 work of the cortical cytoplasm; but if this is so, we shall have to await 

 the development of new techniques of investigation before we can learn 

 much about them (Fig. 2.7). 



'MmiP 



Figure 2.7 

 Three fundamentally different types of egg structure. On the left, a 'mosaic' 

 egg with locahsed regions of cytoplasm (the oligochaete, Tnhifex, after 

 Penners). Centre the echinoderm egg, with a general polarity, indicated by 

 the dark and light circles, and two opposing gradients, shown by the plus 

 and minus signs (after Harrison). Right, an amphibian egg, with a vegeta- 

 tive-anmia gradient dependent on yolk concentration! and a cortical 

 gradient falling off from the position of the future blastopore. (After Dalcq 



and Pasteels.) 



In spite of the fact that our knowledge of these structural properties of 

 the egg-cell is so slender, one must not overlook their fundamental im- 

 portance. When we discuss the eggs of the different kinds of animals, we 

 shall m every case fmd that the eventual origin from which the whole 

 later development sprmgs is the orderly arrangement of essential parts of 

 the ovum. We must therefore enquire a little more deeply how this 



