JENNINGS: DEVELOPMENT OF ASPLANCHNA HEEEICKII. 87 



PART SECOND. -DISCUSSION OF MATTERS BEARING 

 UPON THE MORPHOLOGY OF THE ROTIFERA. 



Our knowledge of the development of the Rotifera is due chiefly to 

 the work of Zeliuka ('91). This author has given a full and careful 

 description of the development of Callidina russeola Zel. from the egg to 

 the adult form, with a briefer, but still extended, comparative account of 

 the development of Melicerta ringens. Earlier works on the embryol- 

 ogy of the Rotifera are due to Salensky ('72), Joliet ('83), Zacharias 

 ('84), and Tessin ('86); but all of these works are incomplete and in 

 many respects inaccurate, so that they have been almost completely 

 superseded by the work of Zelinka. In discussing the development of 

 Asplanchua I shall therefore restrict myself chiefly to a comparison with 

 the results of Zelinka, drawing upon the accounts of other authors only 

 where there is special occasion. 



Since my work has been done primarily from the standpoint of cyto- 

 mechauics, and not with regard to the morphology of the Rotifera, it has 

 an entirely diflerent aim from that of Zelinka. It thus results naturally 

 that, in giving an account of the bearing of my studies on questions 

 relating primarily to rotifer morphology, emphasis must be laid chiefly 

 upon the points in which my results difl'er from those set forth in 

 Zelinka's paper. The plan of my work required a more minute -study of 

 the cleavage than was demanded for Zelinka's purposes, and as a natural 

 result I shall be compelled to criticise his account of the cleavage in 

 regard to certain details. Furthermore, it will be necessary to show 

 that Zelinka has been inconsistent in his account with regard to the 

 place where the polar cell is foi'med, and hence is mistaken in his state- 

 ment of the axial relations of the egg and embrj'o in the Rotifera. But 

 all these are matters of detail, not affecting in an}'- important way 

 Zelinka's general conclusions, and I wish to say at the beginning that I 

 fully appreciate the thoroughness and excellence of Zelinka's researches 

 upon this difficult group, and make the criticisms and corrections con- 

 tained in the following pages in no spirit of disparagement. 



Asplanchua Herrickii de Guerne, the form upon which my studies have 

 been made, is not closely related to any of the species of Rotifera whose 

 development has been previously described. Callidina and Melicerta, 

 investigated by Zelinka ('91), belong respectively to the groups Bdelloidea 

 and Rhizota of Hudson and Gosse ('86). Rotifer and Philodina, studied 

 by Zacharias ('84), belong also to the Bdelloidea. Brachionus, investi- 



