ANNELIDA 263 



I 



eggs are deposited inside the tube, and are found here be- 

 tween it and the body-wall. Similar to this is the brooding 

 in the genus Capitella, in which one finds the eggs firmly 

 attached like a mosaic, on the inner surface of the tube. 

 Other tubiculous worms attach their eggs to their habitations 

 on the outside ; thus in Sabella lucuUaria the eggs, which are 

 enveloped in a slimy mass, form a thick ring around the tube 

 of the parent. Many PoJychseta deposit their spawn in the 

 form of large gelatinous packets or clumps (Aricia, Psygmo- 

 hranchus) ; others discharge the eggs into the sea-water 

 without any other protection than the egg-membrane 

 (Ertpomatus, Pomatoceros et al., likewise Polygordius). In 

 such forms artificial fertilization can be employed with 

 success. 



Cleavage is unequal, but in some forms may approach very 

 near to the equal type (Pomatoceros according to v. Deasche). 

 In the latter case it produces a aeloblastula, the entodermic 

 part of which can be distinguished by the greater thickness 

 {Sahellaria, Aricia), or an epibolic gastruJa is formed (Nereis, 

 Psygmobranchtis) . The Polychgeta, which have been studied 

 with this object in view by Hatschek, Goette, v. Drasche, 

 Salensky, and other investigators, offer all transitions between 

 the different types of cleavage, and correspondingly the form 

 of the gastrula also varies from a typical invagination to an 

 epibolic gastrula. In Terebella Meckelii, for instance, we find 

 a blastula with the wall thickened on one side, the cavity of 

 which soon becomes filled by the intruding macromeres, so 

 that we now have before us a so-called sterrogastrula (Salen- 

 sky). [Wilson (Appendix to Literature on Annelida, No. 

 XXVII.) has recently given a very detailed account of the 

 early stages of development in Nereis, especially of the cleav- 

 age, in which the fate of the individual cells is established 

 with great precision. — K.] 



As an example of the embryonic development of a Poly- 

 chaete, we select that of Eupomatus (according to Hatschek). 

 The spherical egg is divided by means of the first two meri- 

 dional planes and succeeding equatorial plane of division 

 into eight blastomeres of almost equal size. Soon, however, 

 the divisions at the animal pole take place more rapidly than 



