THE GAMETES 



41 



and is therefore rather difficult to investigate. Many eggs contain, besides 

 food reserves, a number of other granules, which may be grains of pig- 

 ment, or mitochondria at which enzyme activity occurs, or may be of a 

 so-far-undetermined nature (Fig. 2.6). We shall have to describe later eggs 

 in which different regions of cytoplasm can be recognised by the different 

 types of granule which they normally contain (e.g. in ascidians p. 106). But 

 again, at least in many cases, the larger granules do not themselves deter- 

 mine the development of the different regions, since, like the yolk, they 

 can be shifted by centrifuging without affecting it. On the other hand. 



A. Genetic bast's: 

 I Chromosomes 

 'iTh/monucttlc icid 



Figure 2.6 



The internal constituents of the oocyte of the snail Limnea, as seen after 



centrifugation. On the right the distribution of the various components is 



shown diagrammatically, on the left is a drawing of an actual section. 



(After Raven 1948, and Bretschneider and Raven 1951-) 



we shall find examples (again for instance in ascidians) where, if the centri- 

 fuging moves the actual clear ground substance of the cytoplasm, togedier 

 with the ukra-microscopic granules, the ensuing development is pro- 

 foundly altered. In such cases, it is clear that the different parts of the egg 

 cytoplasm are endowed with different developmental properties. 



In other eggs, such as those of echinoderms, one can move even the 

 internal cytoplasm around a good deal without interfering with develop- 

 ment. But even in such cases, there are probably always some structures 

 in the egg whose integrity is essential for the formation of a normal 

 embryo. The most frequent location of this essential structure is the 

 surface layer. The cytoplasm here is generally stiffer and more elastic than 



