JEXXINGS: DEVELOPMENT OF ASPLAXCHXA HERPJCKII. 103 



Sections were made of numbers of eggs, but optical sections are much 

 more instructive, permitting exact orientation and revealing the struc- 

 ture fully as well as actual sections, so that most of the figures in which 

 sections are represented were made from optical sections. 



In the descriptive portion of this paper, details have been given of 

 the movements of asters, nuclei, and other cell contents, as well as 

 of the cells as units. As the entire account was gained from a study 

 of preserved material, the question is a justifiable one, — Is there suffi- 

 cient evidence that the movements actually occur as above described, or 

 are the stages figured and described merely chosen at will from a mass 

 of material and arranged arbitrarily in series ] 



The number of eggs used in determining the course of events in a 

 given cleavage has been stated in several cases in the text. Thus, 31 

 eggs were studied containing more than one and less than five cells; 

 42 containing more than seven and less than sixteen, etc. In all, more 

 than 250 eggs from Asplanchna Herrickii and 50 from Asplanchna prio- 

 donta, between the single cell stage (Fig. 1) and the stage containing 

 five entoderm cells (Fig. 83), were mounted in glycerine and studied. 

 Each egg, of course, came necessarily from a different individual, — 

 since, where two embryos were present in the same adult, one at least 

 had passed to a stage in the formation of organs. Of many of these 

 eggs examinations were made which may be called exhaustive ; i. e. 

 every cell with its nuclear conditions was carefully figured. Thus, 

 from the egg of which Figure 68 (Plate 8) gives one view, at least 

 twenty drawings were made, though but one is shown in the plates. 

 The figures given, therefore, represent by no means even a considerable 

 part of the evidence upon which the description is based. After de- 

 termination of the exact order of events, drawings of typical cases were 

 selected for illustrating the paper. 



The determination of the sequence of the stages observed is greatly 

 lightened by the almost entii-e constancy in the relative order of events 

 in the different cells. Very slight variations occur in regard to certain 

 processes, as in the case of the migration of the cloud of gi-anules, as 

 mentioned in the Explanation of Plates, under Figure 51. But, in gen- 

 eral, a number of eggs representing a series of events in a given cell 

 show corresponding series of events in the other cells. It is not neces- 

 sary, therefore, to rely upon the conditions within the cell under ex- 

 amination for determination of the sequence of stages in this cell. 

 Even this would probably be possible, however, from the fact that the 

 nucleus in any cell increases in size steadily from the time the cell is 



