I20 CLEAVAGE AND DIFFERENTIATION 



have been extruded. Here, and in various other Annehds, the 

 material for the pole-plasms and polar lobes appears to have been 

 previously distributed over the whole surface of the tgg, and to 

 some extent in the interior. This follows from the fact that all and 

 sundry unfertilised egg-fragments of Chaetoptenis subsequently 

 fertilised are capable of developing into normally formed miniature 

 larvae,^ whereas the polar lobe in later stages is sharply localised. 



Further evidence of a rearrangement of materials as a result of 

 fertilisation is provided by other experiments in which the develop- 

 ment of egg-fragments is studied. In the Nemertine Cerehratiilus, 

 for instance, such experiments show that there is a progressive in- 

 crease, from before fertihsation, to the onset of the first cleavage, 

 in the restriction of the potencies of animal and vegetative regions. 

 The animal region becomes progressively less able to produce 

 digestive tract and larval lappets, while the proportion of vegetative 

 fragments which produce an apical organ becomes smaller, during 

 the period in question.'^ 



A remarkable fact, whose precise interpretation is not clear, is 

 found in De7italmm. If the unfertilised tgg is cut across, latitu- 

 dinally or obliquely, and then fertilised, the vegetative portion seg- 

 ments as a whole, with a polar lobe usually of correct proportional 

 size.^ The resulting larva also has a correctly proportioned apical 

 organ and trunk. It will be remembered that when a CD blasto- 

 mere is isolated, it forms a polar lobe as large as in the whole tgg, 

 and the larva is disproportionate. A further remarkable fact is that 

 when an already fertilised egg is cut so as to produce an enucleate 

 vegetative fragment, though this does not cleave, it will protrude 

 its polar lobe synchronously with the first division of the nucleated 

 portion : the polar lobe is of the same size as in an intact Qgg. Some 

 irreversible change concerning the quantity of material in the polar 

 lobe must take place at fertilisation. 



Further light on the mechanism of formation of polar lobes is 

 thrown by experiments on t\\^^lo\\u?,c Ilyanassa. Here, a polar lobe 

 is protruded four times : when the first and second polar bodies are 

 forming, and at the first and second cleavages. At its first ap- 

 pearance, the degree of protrusion is extremely slight ; at its second, 

 moderate ; while at its last two appearances, it is very marked, and 



^ E. B. Wilson, 1Q29. ^ Yatsu, 1910. ^ E. B. Wilson, 1904 a. 



