1904.] NATURAL SCIEXCES OF PHILADELPHIA. 689 



Korschelt studied the living egg and described the manner in which 

 the eggs were laid, the early stages of the cleavage, an epibolic gastrula- 

 tion, and observed that at hatching the young Dinophilus closely 

 resembles the adult. Repiachoff's paper is more complete as regards 

 the embryology, giving numerous figures of the cleavage and sections 

 of the gastrula. Two important discoveries are to be attributed to 

 this investigator, viz. : That the mesoderm arises from a pair of meso- 

 blast cells situated posterior to the blastopore, and that a considerable 

 portion of the ectoderm of the adult arises from a pair of large cells 

 situated at the posterior end of the embryo and derived from the largest 

 cell of the 4-cell stage. Schimkewitsch follows along the same lines 

 as Repiachoff, and does little more than confirm the latter's results, 

 though the figures given by Schimkewitsch are much in advance of 

 those given by RepiachofT. These authors, Korschelt, Repiachoff 

 and Schimkewitsch, attempted to compare the cleavage of the Dino- 

 philus ovum wdth that of the rotifers and consequently failed to prop- 

 erly interpret it. 



In view of the fact that so little is at present known concerning the 

 early development of this form, the evidence as to its relationships 

 being principally anatomical, it has seemed highly desirable to study 

 the embryolog}^ thoroughly, from the earliest cleavage on. 



The present paper is concerned almost wholly with the cell-lineage. 

 This has been done for two reasons : first, because such a careful study 

 of the cleavage as is involved in a study of the cell-lineage gives a firm 

 and secure basis for work on the later development; and second, 

 because the study of the cell-lineage of mollusks and annelids has 

 brought to light such striking resemblances that there can scarcely 

 be any doubt that they are of phylogenetic significance. 



II. — Material and Methods. 



The species of Dinophilus with which this paper is concerned has 

 not been determined with certainty. Both sexes correspond closely 

 to the description given by Korschelt (1882) for the species found by 

 him in aquaria at Freiburg, and named by him Dinophilus apatris. 

 Repiachoff (1886) has supposed that this species is identical with D. 

 gyrociliatus 0. Schmidt. The individuals of the species found in the 

 aquaria at the Universit}^ of Pennsylvania agree with D. apatris, and 

 differ from D. gyrociliatus in lacking the last or perianal circle of cilia, 

 and also appear to differ from D. gyrociliatus in another important 

 respect, i.e., in having no segmental organs. E. Meyer (1887) figured 

 for the females of D. gyrociliatus five pairs of nephridia of the type found 



