124 CLEAVAGE AND DIFFERENTIATION 



has been forcibly disarranged by the centrifuge, taking advantage 

 of the different specific gravities of the various egg-contents, but 

 each part of the mosaic continues its predetermined course. In this 

 way, organ-forming substances have been shown to be present in 

 the fertiHsed egg, and respectively responsible for the formation 

 of ectoderm ("ectoplasm"), endoderm ("entoplasm"), neural 

 plate ("neuroplasm"), notochord ("chordoplasm"), muscle fibres 

 ("myoplasm"), and mesenchyme ("chymoplasm")^ (%s. 56, 59). 



It is thus clear that the fertilised egg of the Ascidian is already 

 a highly complex mosaic of chemo-differentiated stuffs, and we may 

 now turn to the experiments in which the developmental potencies 

 of fragments of the unfertilised egg have been tested. 



Latitudinal halves of unfertilised eggs of Ascidiella, subsequently 

 fertilised, show that there is already at this stage a differential re- 

 partition of potencies along the egg-axis. The larvae obtained may 

 be deficient in one or more kinds of tissue according to the level of 

 the cut: myoplasm can be separated from chymoplasm, neuro- 

 plasm from chordoplasm, the former in each case being situated 

 nearer to the animal pole. The various substances must, therefore, 

 occupy different levels.^ 



In view of the rigid mosaic behaviour of isolated blastomeres, and 

 of the definite localisation of substances at fertilisation (as tested 

 by the centrifuge experiments), the further result may seem sur- 

 prising that meridional halves of unfertilised eggs, subsequently 

 fertilised, may give rise either to apparently normal and sym- 

 metrical larvae, or to lateral half-larvae. The former type appear to 

 provide a case of regulation, which would be remarkable in such a 

 form as an Ascidian. These results can, however, be explained on 

 the view that the various organ-forming substances in the unfer- 

 tilised egg occupy circular zones at particular levels surrounding 

 the egg (or possibly crescentic zones, the horns of which quite or 



^ Careful analysis has shown that the visible prelocalised substances in Styela, 

 such as the mitochondria, which impart the yellow colour to the region destined 

 to give rise to muscles, are not themselves morphogenetic substances. Muscles 

 can develop without mitochondria. The various regions differ in the consistency 

 of their cytoplasm, and it is these sharply marked off differentiated regions which 

 appear to constitute the true organ-forming substances. The mitochondria and 

 other gross differences are symptoms, not causes (see Duesberg, 1928). This 

 question of the relation of organ-forming substances to raw materials will be 

 discussed in Chap. vii. 2 Dalcq, 1932. 



