CYTOLOGY OF REPRODUCTION IN ANIMALS 129 



In the foregoing examples it is evident that there is some correlation 

 between the type of cleavage and such visible features as the location of 

 3^olk. The correlation is so far from complete, however, that this featun; 

 cannot be regarded as more than a contributing cause of cleavage pat- 

 terns. Exhaustive studies have shown that the positions assumed by the 

 cleavage spindles and hence of the resulting partitions are determined 

 mainly by some fundamental protoplasmic organization which it is not 

 yet possible to describe. 



A most important aspect of cleavage is the relation it bears to the 

 internal differentiation of the embryo. It is evident that differentiation 

 has proceeded much further in some animals than in others at a given 

 stage of cleavage. It has long been known that in a coelenterate (Cly(ia) 



Fig. 95. — Embryo of rabbit in eight-cell stage; five of the cells (blastomeres) visible iii the 

 .section. {Courtesy of General Biological Supjily House, I tic., Chicago.) 



one of the first 16 blastomeres may produce a complete embryo, whereas 

 in a ctenophore (Beroe) it has been observed that the larva is incomplete 

 if a portion of the egg's protoplasm has been removed even before the 

 first cleavage division. In cases like the latter the egg, before its cleavage 

 or even before its fertilization, may have a definite promorphology, i.e., 

 it has developed an internal organization which in some manner fore- 

 shadows the morphology of the young embryo. Hence to the three 

 processes which sometimes follow each other so closely- as to overlap — 

 meiosis, syngamy, and cleavage — we may now add a fourth, embryonic 

 differentiation. 



In some eggs an internal differentiation can be detected in the pattern 

 assumed by certain visible substances. This pattern may be cut up in 

 various ways by the successive cleavage furrows, the ability of any 

 isolated blastomere to produce a whole embryo or only a part of one 

 being in some measure dependent upon the elements of the pattern it 



