A STUDY IJST MORPHOLOGY. 
67 
long under observation (1 hour 45 minutes) that its development was arrested at this 
point; but another egg in this stage of development was seen to pass into the resting 
condition, as shown in fig. 9, and it then remained quiet for about fifteen minutes, 
showing no external indications of change during this time. 
At the end of the third period of rest the spherules again became prominent, so that 
the outline of the egg was exactly like that of fig. 8, and the egg entered upon the 
fourth period of activity, soon dividing into sixteen spherules (fig. 10), arranged around 
a segmentation cavity. 
In about twenty-five minutes from the beginning of this period of activity the 
spherules began to flatten down once more, and. the egg passed into the fourth resting 
stage, but it was not observed beyond this point. 
The alternation of activity with rest was observed at much later stages, but after 
the gastrula invagination makes its appearance the cells of the endodermic portion of 
the egg do not undergo active change at the same time with those of the ectoderm, 
and the egg has one set of periods of activity for each layer. As development goes on 
the periods of rest grow longer and the periods of activity shorter, and the spherules do 
not flatten down while at rest. 
The egg which is shown in optical longitudinal section in fig. 16 was in the field of 
the microscope for nearly twenty minutes, while I was examining another specimen. 
An occasional look at it showed that it was not changing, but at the end of this time 
I noticed that the outer ends of the ectoderm cells directly opposite the orifice of 
invagination were notched, as is shown in the figure. Activity spread in all directions 
from this point, and in less than five minutes all the cells were notched, and those 
nearest the centre of the area of activity were perfectly divided into halves. In about 
five minutes more all the ectodermal cells had divided, and this layer had the 
appearance shown in fig. 17—which, however, was drawn from another specimen. 
This last egg remained in the condition shown in the drawing for fifteen minutes 
from the time it was first observed, and a movement of the appendage to which it was 
fastened caused it to roll over and present its formative pole for examination before the 
beginning of the next period ot activity, which is shown in surface view in fig. ] 8. 
The manner of division was simply a repetition of that which has just been described. 
The cells nearest the centre of the formative area became notched, and then divided 
into halves; and the activity gradually spread over the egg in all directions, until, in 
a few minutes, all the cells which were visible were at some stage of division. 
The rapidity and uniformity with which this change spread over the egg rendered it 
an extremely interesting and impressive sight, and I know of no other case in which 
segmentation is so perfectly regular at such an advanced stage of development. 
The activity did not affect the endoderm cells in either of these cases, but at a later 
stage (fig. 20) they were seen to be in an active segmenting condition at a time when 
the ectoderm cells were at rest. I was not able to keep this egg alive long enough to 
w r atch the completion of the process, for it had been under the microscope for some 
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