No.401.] DEVELOPMENT OF PENNARIA TIARELLA. 399 
rhythmic relations disturbed to such an extent that this very 
erratic and anomalous type of segmentation has resulted. May 
it not be within the line of possibilities that the peculiar meth- 
ods and conditions of growth and maturation of these and some 
similar eggs have in some way been the occasion of the disturb- 
ance?' While only a suggestion, yet it seems not without the 
range of possibilities. | 
In Pl. III, Figs. 1-7, are shown conditions not uncommonly 
met with, which are specially interesting in that from one such 
were derived spontaneously two perfect embryos, a fact, so far 
as I am aware, quite unusual, if not unique, though Metschni- 
koff ('86) cites a somewhat similar case in the development of 
Oceania armata. It does not seem clear, however, from his 
account whether the cases are quite similar, certainly not in 
the details of the cleavage. Following the first cleavage in 
these eggs, during which the blastomeres become widely sepa- 
rated, the two halves proceed to develop quite independent of 
each other and show no disposition to reunite until the cleav- 
age is quite advanced, apparently to the point of completion or 
nearly so, when usually they gradually approximate and finally 
fuse into a typical morula and develop into a normal embryo. 
In at least one case a specimen which showed this aspect in a 
very marked degree at an early stage gave rise to two perfect 
larva, though of small size. The specimen was carefully iso- 
lated in a large watch glass and set aside as a test case, and the 
next morning the larvae were found in perfect condition, as indi- 
cated. Several others of similar character were subsequently 
isolated in the same way, but in only the one case did this 
spontaneous division show itself conclusively. That such 
cases, though rare, are not strange among these organisms 
may be very well conceded, specially when the peculiar phe- 
nomena associated with the early development are familiar. 
VI. COMPLETION OF SEGMENTATION AND FORMATION OF THE 
PLANULA. 
Following the more conspicuous and anomalous aspects 
already considered, the egg gradually assumes a nearly spher- 
