TYPES OF CLEAVAGE 



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cerning the cleavage phenomena in the turtle. The information given below 

 is to be correlated also with the developing pigeon's egg.) 



The germinal disc (blastodisc) of the hen's egg at the time that cleavage 

 begins measures about 3 mm. in diameter. Its general relationship to the 

 egg as a whole is shown in figure 157A. 



1) Early Cleavages. The first cleavage furrow makes its appearance at 

 about four and one-half to five hours after fertilization at the time when the 

 egg reaches the isthmus of the oviduct (figs. 155A; 157C, D). The first 



Fig. 155. Cleavage in the chick blastoderm, surface views. (C after Olsen. '42; the 

 rest after Patterson, MO.) (A) First cleavage is approximately meridional. (B) Second 

 cleavage is at right angles to first. (C) Third cleavage planes are vertical as indicated 

 and approximately parallel to one of the other cleavage planes. Considerable inequality 

 may exist at this time. (This figure slightly modified from original.) (D) Seventeen- 

 cell stage. Observe central and marginal cells. (E) Stage approximating 32-cell con- 

 dition. (F) Surface view of 64-cell stage; 41 central and 23 marginal cells. (G) Surface 

 view of blastoderm in lower portions of oviduct; 31 marginal and 123 central cells. (H) 

 Later blastoderm, showing 34 marginal and 312 central cells. 



