378 
EDWIN LINTON 
figured (figs. 1 to 4, 7, 20 to 30, 37 to 39, 41). It would extend 
this paper beyond its intended scope to describe them in detail. 
Briefly the normal course of events in the development of the 
young seems to be as follows: 
1. The egg as it is liberated from the ovary-vitellarium is char- 
acterized by having a relatively large amount of yolk in which 
there is a large, clear nucleus with a distinct nucleolus (fig. l,h). 
How the capsule is acquired was not made out. 
2. The nucleus divides, and sooner or later there is a cleavage 
of the resulting mass of cells into two divisions ; or, che two nuclei 
may separate at the completion of the first segmentation (fig. 
3, / and h) So far as I have been able to interpret the evidence, 
the embryos which are enclosed in the same capsule seem to have 
developed from the same ovarian ovum. 
3. Each division is soon seen to be made up of two kinds 
of cells, (a) Small and numerous, massed at one pole, which 
ultimately becomes the head of the young worm. There is 
also a narrow layer of these small cells around the periphery 
(b) Large, globular cells, relatively few in number These lie in 
the posterior region, and continue with little change in the 
young worms (fig. 41, e and d). They are probab^ yolk-cells. 
4. The small peripheral cells soon give rise to a ciliated epithel- 
ium. A body-wal] about as thick as that of the adult appears, 
pigmented eye-spots are developed and the rudiment of a pharynx 
is seen The egg-membrane persists as a capsule in which the 
pair of young worms are confined long after they are capable of 
active movements. At first they lie parallel, with their heads 
and tails in corresponding positions. Later the head of the one 
is usually beside the tail of the other. They keep up a constant 
movement round and round, sometimes in the same direction, 
but often moving independently of each other. Usually the two 
young and the fluid in which they are immersed are the sole 
contents of the capsule ; at times there may be one or more masses 
of yolk also included in the capsule. There seem to be individual 
characteristics in this respect, the course of development of the 
young in one adult being quite different from that in another (figs. 
- 2 and 3). A few cases were noted where there were three em- 
