THE BEGINNING OF DEVELOPMENT 37 



eggs are said to have partial or discoidal cleavage, as 

 opposed to the total cleavage of the less yolky types. 

 The total cleavage, as we have said, may be equal 

 or unequal {Fig. 6), and further, it may be quite at 

 random, forming a confused mass of cells ; but often, 

 particularly in the "total unequal" cleavages, it 

 follows regular rules, so that the resulting cells are 

 arranged in a definite pattern. An example of this, 

 in the sea-urchin's tgg, is described later (see p. 73). 

 The cleavage cuts up the large egg-cell into 

 smaller cells, each of which, we can imagine, is 

 more completely under the control of its nucleus 

 than the unwieldy ^gg could be. It would be easy 

 to suppose, and at one time it was supposed, that 

 the nuclei divide unequally during the cleavage, so 

 that the nuclei are unlike each other, and cause the 

 cells in which they lie to develop in different ways 

 into the various organs of the adult. But as a matter 

 of fact this supposition is quite wrong : the cleavage 

 nuclei are all alike. A very neat proof of this has 

 been given by Spemann (Fig. 7). He tied a hair 

 round a fertilized newt's tgg, pinching it into a 

 dumb-bell shape, so that the nucleus lay at one end 

 and the other end had no nucleus. The end with 

 the nucleus cleaved, while the other end did not. 

 After several cleavages the knot was loosened and a 

 nucleus, whichever happened by chance to lie 

 nearest, allowed to pass through the bridge between 

 the two ends. The second end now started to cleave, 

 and it developed, not into any special part of the 

 embryo depending on which nucleus it got, but into 



