CLEAVAGE AND DIFFERENTIATION II3 



cleavage mechanism to particular blastomeres, and therefore later 

 distributed to particular and circumscribed regions of the embryo, 

 are found in other Molluscs and Annelids. A polar lobe very 

 similar to that of Dentalium is found in the Gastropod Ilyanassa 

 and the Polychaetes Chactopterus and Myzostoma. In Ilyanassa, if 

 the polar lobe is removed, no mesoderm is produced and the larva 

 is abnormal in form. Isolated blastomeres give rise to incomplete 

 embryos which die before reaching the larval stage.^ No experi- 

 ments appear to have been performed on Chaetopterus and 

 Myzostoma, but there is every reason to think that the polar lobe 

 in them plays a similar part. 



In the Oligochaete Tubifex, the Q^g possesses so-called pole- 

 plasms — clear areas of cytoplasm at the animal and vegetative poles. 

 By means of extremely unequal cleavage, these are entirely re- 

 stricted, first to blastomere CD, and then to blastomere D. The 

 two pole-plasms then unite near the centre of the cell. At the next 

 cleavage the united pole-plasms remain entirely within the macro- 

 mere (i/)), but a portion of them passes to the first (ectodermal) 

 somatoblast, 2 d. The remainder, which is left in 2-D, passes at the 

 fifth cleavage entirely into 3Z), and then at the sixth into the second 

 (mesodermal) somatoblast, 4^. The course of events in the leech 

 Clepsine appears to be the same. 



In Tubifex, none of the blastomeres AB, A, B, or Cis capable by 

 itself (the unwanted blastomeres being killed by uhra-violet light) 

 of developing into anything approaching a complete embryo; 

 blastomere D, however, can develop into a complete and properly 

 proportioned embryo.- 



Other experiments on Tubifex have confirmed and extended 

 these results. By certain methods (application of heat, or depriva- 

 tion of oxygen) the first cleavage can be made to take place equally 

 instead of unequally, and in this case both blastomeres of the 2-cell 

 stage possess equal amounts of the pole-plasms. From such eggs, 

 double monsters (of the cruciata type, see p. 156) are produced. 

 It would appear that when the time comes for the formation of 

 micromeres, each set of pole-plasms will give rise to a set of 

 somatoblasts (one ectodermal and one mesodermal), and these will 

 differentiate independently into the main organs of the trunk.^ 



1 Crampton, 1896. 2 Pgnners, 1925. ^ Pgnners, 1924. 



HEE C 



