596 
T. H. MORGAN 
the yolk and pigment may pass entirely to the small cell, or, in 
other eggs, the small cell may contain the oil, or else only the clear 
protoplasm of the egg. 
The development of the swimming trochophore (plate 3, fig. I) 
takes place in about 24 hours. If the eggs are ripe and the condi- 
tions favorable, the trochophore begins to flatten during the 
next 12 hours and after 48 hours (plate 3, fig. VI) has become a typ- 
ical veliger (plate 3, fig. XII, XIII) . By no means all sets (eggs from 
one individual) reach this stage. Even in the best part of the 
season not more than half of the sets become veligers and the 
others remain in the swimming trochophore stage until they die. 
This is the more surprising since the eggs segment with the pre- 
cision of clockwork in nearly all of the sets, and the early stages 
of development appear to be normal. 
This failure of many sets to produce normal embryos has added 
to the problem an unforeseen difficult^' , which was greatly increased 
when it was found that centrifuged lots rarely produced normal 
veligers, although here too the early stages were entirely normal. 
Occasionally, however, a centrifuged set would produce many 
veligers and sometimes as many as its normal control. This 
result showed that centrifuging, in so far as it segregated the 
visible materials, was not th-e cause of the failure of most sets to 
develop. Convinced by this evidence that some other condition 
than the redistribution of the materials was responsible for this 
great majority of failures, I have spent a large part of two summers 
in the attempt to discover why the centrifuged eggs usually go 
badly, and why they occasionally develop as well as the controls. 
I have found an answer to these questions and the means of avoid- 
ing the trouble, as the following description of the experiments 
will show. 
Polyspermy is a common cause of abnormal development. 
Eggs of an individual were put into small dishes and each set fer- 
tilized by the sperm of a different male. All produced abnormal 
embryos. As a control some of the eggs were put into a large 
dish of water and a little sperm added. They developed normally 
aven when they were transferred to smaller dishes after fertiliza- 
tion. Sea water from the ocean gave better results than sea water 
