126 CLEAVAGE AND DIFFERENTIATION 



almost meet). Any meridional half will thus contain a portion of 

 all the necessary substances. However, the cytoplasm of the un- 

 fertilised egg appears to be already endowed with a plane of bilateral 

 symmetry, and if the cut through the egg is made at right angles to 

 this plane, the resulting half-egg will be able to form a complete 

 and symmetrical larva. But if the cut coincides with the plane of 

 bilateral symmetry, the half-egg will give rise to an asymmetrical 

 half-larva.^ 



At this stage, therefore, regulation is possible in some cases, owing 

 to the fact that the organ-forming substances are localised in such 

 a way that egg-fragments may contain portions of all of them. Sub- 

 sequently, however, at fertilisation, the localisation becomes more 

 restricted, the circular bands or crescents become reduced to 

 smaller crescents, the horns of which do not extend more than a 

 quarter of the way round the egg on each side, and this, together 

 with the high viscosity, effectively prevents regulation. 



It is worth stressing that in Dentalium, the CD blastomere pro- 

 duces a larva which, while showing disproportion in regard to the 

 organs derived from the polar lobe, appears to have undergone 

 regulation round the major axis, thus becoming bilaterally sym- 

 metrical. Similarly, in Amphioxus a lateral 1/2 blastomere produces 

 a bilaterally symmetrical larva. Both in Bero'e and the Ascidians, 

 however, 1/2 larvae preserve the laterality of the blastomere from 

 which they arose. Here again, it must presumably be the high 

 viscosity of these eggs which has prevented the rearrangement round 

 the main polar axis of materials needed for regulation. 



§7 

 Returning to the question of the relation of cleavage to differentia- 

 tion, it may then be said that the part which cleavage plays is only 

 indirect. Cleavage is a process whereby the single-celled fertilised 

 egg is split up into a number of separate cells whose differing 

 qualities depend upon factors which are originally independent of 

 cleavage, and concern the viscosity of the Qgg and the time of 

 chemo-differentiation of its cytoplasm. 



In this connexion, we may refer to the very interesting case of 

 the insect egg. Here, cleavage of the nucleus begins and continues 



^ Dalcq, 1932. 



