114 CLEAVAGE AND DIFFERENTIATION 



Similarly, in Chaetopteriis, it has been found possible to make 

 the first cleavage take place equally instead of unequally, by tem- 

 porary compression exerted after extrusion of the second polar 

 body and released when the first cleavage plane has cut half-way 

 through the egg. In this case, both the blastomeres of the 2-cell 

 stage receive a half of the polar lobe, and the result is the formation 

 of double monsters (likewise of the cruciata type). If, however, the 

 1/2 blastomeres of such eggs are isolated, each can give rise to a 

 single whole embryo. ^ Double monsters have also been found in 

 the leech Clepsine^^ where they are probably due to equality of 

 cleavage divisions. It is interesting to note in this case that the 

 spiral cleavage of the right-hand member is reversed. 



It further appears from experiments on Clepsine, in which the 

 method of damaging small areas of the unsegmented egg was 

 employed, that the animal pole-plasm is necessary for cleavage to 

 occur at all. When only the vegetative pole-plasm has been 

 destroyed, cleavage is more or less normal except that it is delayed 

 in the D quadrant. Both the somatoblasts (2^ and 4^) are formed, 

 though /[d contains more yolk than normal. However, although 4^ 

 produces the rudiments of mesodermal germ-bands, these are in- 

 capable of differentiation, and the embryo dies. It is not known 

 whether the ectodermal germ-bands (derived from zd) could 

 differentiate, as this only occurs at a later stage. 



The cleavage-pattern, even in the absence of vegetative pole- 

 plasm, is thus predetermined down to the formation of the 

 rudiments of mesodermal germ-bands of typical appearance. But 

 the chemo-differentiation of these to definitive mesoderm is 

 dependent on the presence within them of an organ-forming sub- 

 stance derived from the vegetative pole-plasm.^ 



We may here draw attention to the work on the limpet Patella,'^ 

 which demonstrates the extraordinary restriction of potency shown 

 by the micromeres in forms with spiral cleavage. In Patella, no 

 polar lobe is formed, but, as in Tiihifex, the potentiality for pro- 

 ducing mesoderm is restricted to the D quadrant. An isolated 

 micromere of the first quartet (i.e. a 1/8 animal blastomere, la 

 to id) produces a purely ectoblastic structure with cells typical of 



1 Titlebaum, 1928. '" Muller, 1932. 



3 Leopoldseder, 193 1. * E. B. Wilson, 1904B. 



