ASPLANCHNA EBBESBORNII (ROTIFER) 419 



entire animal (trochal region, body, and foot), develop simul- 

 taneously, hence the position taken by the early cleavage cells to 

 meet the future needs in the formation of the adult structures. 



In Dinophilus the larval or trochophoral development is ac- 

 centuated, and the future adult body at first exists in a concen- 

 trated form in a certain cell or group of cells at the future 

 posterior end of the animal. The end result in either case is a 

 distinctly formed animal, which exhibits many points that are 

 common to both. The development of Dinophilus, however, is 

 more like that of the annelids than of the rotifers, and could be 

 considered as intermediate between the two groups of animals. 

 Further, when considered from the standpoint of development, 

 the rotifers exhibit many points of resemblance to the annelid 

 larva, and no doubt represent a primitive type preserved as 

 modifications in the annelid larva, but they cannot be regarded 

 as an ancestral type from which the annelids sprung, but rather 

 a form which represented the annelids at one period in their 

 phylogeny. 



Hence from cleavage and development of the various forms in 

 question, the conclusion is reached that the polyclades, annelids, 

 and rotifers must have originated from a common ancestral 

 form comparable to that of the ctenophore. 



The rotifers, although retaining their primitive condition, have 

 departed somewhat from the common ancestral form and to some 

 degree have become a specialized group. The annelids, on the 

 other hand, have reached a much higher level in their evolu- 

 tionary history and show affinities to the ancestral type during 

 their larval development. 



GENERAL SUMMARY 



The undivided egg of Asplanchna ebbesbornii is nearly oval. 

 Its median longitudinal axis, passing through the polar body or 

 bodies, corresponds to the median longitudinal axis of the 

 future adult. The first cleavage plane is at right angles to this 

 axis and divides the egg into two very unequal cells. The 

 second cleavage occurs at an angle of about 45° to that of the 

 first. It divides the smaller cell equally and the larger very 

 unequally. The larger cell divides first. 



