FISSION IN NEMERTINES. 23 
another, as is shown in fig. 5, through the region p. Here, 
however, we have a further phenomenon: the double row of 
connective-tissue nuclei having been established, the longi- 
tudinal muscles break across between the two rows of nuclei. 
This rupture appears to start just below the epidermis, and 
then travels inwards. But already the epidermis exhibits a 
furrow even at an earlier stage, as at c (fig. 4), so that it is not 
merely the result of a contraction of the muscles. 
The muscles, once ruptured, naturally contract, and leave a 
space which appears in sections to be partially occupied by a 
coagulum (2), though it may bea pre-formed material—part of 
the connective tissue. This appears to be the more likely, as is 
seen at a later stage in the process, as in fig. 6, which passes 
through the middle of the region a. Here the rupture of the 
muscle has gone a step further, and is extending towards the 
opposite side; this figure also illustrates another point—viz. 
that the rupture may commence at one point of a plane and 
extend in all directions in a radial direction, so that while it 
is complete at one side of the body it may not have yet com- 
menced on the opposite side. 
In this figure, for instance, the muscles have shrunk con- 
siderably at the left side; yet, though the rupture is extending 
towards the right, the muscles are still entire below the epi- 
dermis of the right side. By the rupture of the muscles the 
linear arrangement of the connective-tissue nuclei is to some 
extent destroyed. 
Finally, the epidermis, which during this process has become 
thinner, as well as furrowed at the plane of constriction, gives 
way; but the basement membrane still appears to resist the 
rupture, for in fig. 7 there is a distinct membrane (dm.) left 
after shrinkage of the epidermis. We must consider this 
basement membrane, as Hubrecht and others have suggested, 
as a firm skeletal tissue, and its resistance to the process here 
described is only what would be expected. 
When the rupture is complete, or even before it has travellod 
all the way round the worm, the circular muscles come into play 
(right side of fig. 8), and drawing together the margins of the 
