ASPLANCHNA EBBESBORNII (ROTIFER) 417 



is true that the early development cannot be regarded as in- 

 fallible in determining relationships in every case, yet it may 

 be called to aid in solving genetic relationships of questionable 

 forms. 



It is with the above points in view that the early stages in the 

 development of the rotifers, as compared with other forms, are 

 emphasized as being of considerable phyletic importance in the 

 determining of relationships. 



In Asplanchna ebbesbornii, the early development shows such 

 striking and accurate resemblances to that of the annelids, es- 

 pecially that of the fresh-water Bdellodrillus, that it seems 

 almost impossible to think that such minute similarities could 

 have arisen independently in their evolutionary history. 



The shape of the eggs, the position of the polar body or bodies, 

 and the early cleavage stages are almost identical. In the be- 

 ginning with the one-cell stage the position of the cleavage spindle 

 and the direction of the cleavage are the same, dividing the egg 

 into two very unequal cells (figs. 7, 8, 16, and 17). The first 

 cleavage plane occurs at right angles to the median longitudinal 

 axis of the future adult. In the formation of the four-cell stage, 

 the large cell divides first and very unequally, while the smaller 

 cell divides equally. These two cleavages, taken together repre- 

 sent the second cleavage as it occurs in the annelids. The 

 large cell D is posterior, B anterior, C right, and A left, with 

 reference to the median axis of the future individual. The 

 large cell D and its derivatives always divide first. The position 

 of the four macroneres, as in Bdellodrillus, determine the orienta- 

 tion of the future adult organs of the rotifer (figs. 20 to 24). 



The sequence of cleavage stages corresponds to that of the 

 polyclades, nemerteans, and the annelids. Three generations of 

 micromeres are formed, which contribute to all of the definitive 

 ectoderm and the ectomesoderm (larval mesoderm of the anne- 

 lids). After the formation of d^ 3D gives rise to all of the en- 

 toderm and the mesentoderm, including the reproductive organs. 

 In the rotifers, however, the entoderm and mesoderm are sepa- 

 rated at a later stage in the cleavage of 3D, also 3A, 3B and 

 3C remain on the surface in the region of the blastopore and do 



