THE BIOLOGY OF THE CELL SURFACE 



The foregoing general statement on the cleavage-process 

 in animal eggs,, though it makes no pretension to be exhaus- 

 tive, nevertheless covers the essential points on a process 

 which has been the subject of much admirable and pains- 

 taking investigation. In a book like this a comprehensive 

 treatment of cleavage would be out of place, because it 

 demands more space than we can give it. Moreover, a 

 certain advantage obtains here, as with other biological 

 processes, in setting forth a plain and simple statement of 

 salient features that stand as accepted and established facts. 

 Without further discussion we may note the following 

 points concerning the cleavage-process in animal eggs: 



1. Cleavage may involve the whole or only a part of the 

 animal egg. Hence, the pattern varies depending upon 

 the area of the egg which undergoes cleavage; the pattern 

 at the termination of the cleavage-period resulting from 

 total cleavage, as in the egg of a snail, is markedly different 

 from that of its near relative, the squid, which undergoes 

 partial (discoidal) cleavage. 



2. The size of the blastomeres contributes a distinguish- 

 ing feature to the cleavage pattern. Where in developing 

 insect eggs the cell-size has been investigated, it was found 

 that the blastomeres in the superficial cytoplasm show 

 equality in size. This is not true for the cleavage-cells in 

 the eggs of the ink-fish and in other eggs exhibiting discoidal 

 cleavage. In total cleavage the blastomeres may be mark- 

 edly unequal in size. Hence, not only the extent of 

 the egg undergoing cleavage, but also size-differences of 

 the blastomeres in the cleavage-area constitute a factor 

 which determines an egg's cleavage pattern. 



3. In all forms of cleavage, except superficial, the initial 

 splitting up of the egg-substance takes place at that pole 

 from which the polar bodies have formed or will form. In 

 total cleavage the first and second cleavage planes cut 

 through this pole at or nearly at riglit angles to each other. 



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