II CLEAVAGE 49 



First, however, a word on the conditions of there being 

 a system of cleavage at all. 



It is an established physical principle that drops of fluid 

 will only cohere to form a system of drops without fusing 

 with one another if they are coated with a layer or film 

 which is insoluble both in the surrounding medium and in the 

 fluid of the drops themselves. If the former condition be not 

 observed then the drops separate, if the latter condition be 

 absent then the drops fuse. In neither case is there a system. 



A beautiful experiment of Herbst's has shown that the 

 possibility of the blastomeres into which an egg divides 

 forming a coherent system is governed by a like condition. 

 Around and between the blastomeres there is visible in many 

 cases (the Sea-urchin egg for instance) a definite coating film. 

 This is insoluble in ordinary sea-water, but if the egg be 

 placed in an artificial sea-water from which the calcium has 

 been omitted, then the film is seen to become dissolved, and 

 the blastomeres separate. So far, therefore, the blastomeres 

 behave like drops of other fluids. We shall see now that the 

 pattern assumed by the cells in cleavage may also be explained, 

 in some cases at least, by reference to the principles of surface- 

 tension. 



There are three principal patterns of cleavage, the radial, 

 the bilateral (including the iso-bilateral), and the spiral. The 

 chief features of these types have been often described, but 

 may be briefly recapitulated here. 



The radial type of cleavage is distinguished by the fact that 

 more than three, usually four or eight, surfaces of contact 

 between adjacent blastomeres may meet in one line ; for in- 

 stance, in the four-celled stage of a Frog's egg the four surfaces 

 in question intersect in the egg-axis, and so on. The planes 

 of division in this type also either include, are parallel to, or 

 at right angles to the egg-axis. The same must be said of 

 cleavages of the second type, with the addition that they are 

 symmetrically disposed on each side of one plane, often that of 

 the first furrow, as in the Cephalopod egg (Fig. 10), but it may be 

 another plane, for instance in Ascaris the plane including the 

 first four blastomeres. In the iso-bilateral form there are, of 



1963 E 



