ASPLANCHNA EBBESBORNII (ROTIFER) 391 



5. The origin of the various organs cannot be traced back 

 to any definite cell or cells of the early cleavage stages, but can 

 be directly associated with definite regions of the early gastrula. 

 It is true, however, that the products of any one cell A, B, C, 

 or D, in the production of the ectoderm, can be definitely followed 

 and localized in the gastrular ectoderm, but there are no struc- 

 tural differences to mark off these regions. The ectodermal 

 cells of any region possess the same potentiality in producing 

 muscle fibers, regardless of their origin. It is impossible to tell 

 definitely the differentiation of the digestive and reproductive 

 systems until late cleavage. 



6. The derivative of the blastomere B form the ventral, those 

 of D the dorsal, those of A the left lateral, and those of C the 

 right lateral ectoderm of the adult animal. 



7. The brain on the median dorsal side is derived from the 

 ectoderm at the anterior end. The urogenital sinus is formed 

 by a solid ingrowth of the ectoderm on the median dorsal side 

 at the posterior end above the base of the embryonic foot. Its 

 position is considered as being ventral in the adult, due largely to 

 the disappearance of the foot in later embryonic life. The 

 embryo in its early development is curved ventrally, making the 

 dorsal side appear quite long in comparison to that of the ventral. 

 The mouth is ventro-anterior. 



NATURAL HISTORY 



The rotifer Asplanchna ebbesbornii is not very abundant in 

 this immediate locality. The material was collected from small 

 rain pools and placed in aquaria, filled with tap-water. This 

 particular rotifer was first observed in one of the freshly prepared 

 cultures in January, 1916. They persisted about two weeks and 

 disappeared. They reappeared in the following March and 

 continued for two weeks. The rotifers appeared again in the 

 following May. In each instance about two months elapsed 

 between the times of their appearance. This periodicity con- 

 tinued until December, 1918, when this paper was completed. 

 In each cycle of appearance males, females, and resting eggs were 

 formed in about the same proportions. The cultures were kept 

 indoors at laboratory temperature. 



