JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 108 
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 deseriptive portiou 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 sufli- 
cient evidence that the movements actually oceur 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 loast 
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 deseription 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 entire constancy in the relative order of events 
in the different cells. Very slight variations occur in regard to certain 
processes, as in the ease of the migration of the eloud of granules, 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 
