VII 



ANNELIDA 



161 



may say at once that these " vegetative micromeres," as we may term 

 them, give rise to the posterior ectoderm and the external circular 

 muscles. They form in fact part of the trunk blastema. 



The endodermal epithelium does not arise as in other Annelida 

 from the fourth and fifth quartettes of micromeres and from the 

 equal division of the residual macromeres, but by the budding of a 

 single cell from the residual macromere 2B, and of several cells from 

 the residual macromere 4D. The residual macromeres in the 

 quadrants A, B, and C, after having undergone the divisions recorded 

 above, give rise to no more cells ; they become smaller and smaller as 

 development goes on, and are finally absorbed. The cells destined to 

 form the lining of the alimentary canal lie between them, and the 

 remains of the macromeres 



are thus found outside the mac 



alimentary canal ; a contrast 

 to the condition of affairs 

 obtaining in the Ehynco- 

 bdellidae as represented by 

 Clepsine, where the endoder- 

 mal cells are budded from 

 the surfaces of all four macro- 

 meres and surround them. 



Thus, in one group of 

 leeches the endoderm is 

 formed from practically only 

 one macromere, and lies in- 

 side surrounded by the four 

 macromeres, whilst in another 

 group of leeches, the Ehyn- 

 chobdellidae, including 

 Clepsine (Glossiphonia) and 

 its allies (Fig. 120), the 

 endoderm is formed from all four macromeres and lies outside 

 them. We conclude that both forms of development are modifica- 

 tions of a primitive type, such as is seen in Polygordius and most 

 Polychaeta, in which the residual macromeres are directly converted 

 into the endodermal epithelium ; and we are reminded of somewhat 

 similar differences in endoderm formation between Siphonophora and 

 other Hydrozoa, amongst Coelenterata (Chap. IV), and between 

 Planocera and ' Yungia amongst Platyhelminths (Chap. V). 



In Nephelis,' after the endoderm is formed, a transverse row of 

 ten cells can be discerned at the hinder end of the embryo. Of these, 

 two are situated more internally than the rest, and these two are the 

 teloblasts of the true coelomic mesoderm and owe their origin to the 

 division of 4d, the sole member of the fourth quartette to be formed. 

 The remaining eight owe their origin to the division of the four 

 "vegetative micromeres" mentioned above. Of the eight, the two 

 nearest the mid-ventral line are termed neuroblasts, because they 



VOL. I M 



FIG. 120. A fairly advanced embryo of Clepsine 

 (Glossiphonia) seen from behind. (After 

 Whitman.) 



end, cells forming definitive endoderm on the outer 

 surface of mac, degenerating maeromeres.; tb 1 , trnnk 

 blastema of right side ; i& 2 , trunk blastema of left side. 



