528 WILLIAM BATESON. 
Soon after its appearance it consists of amass of loose round 
cells. A cavity next appears in its interior, as though due to 
a disintegration, and after the appearance of this cavity the 
cells bounding it develope into ova (figs. 111 and 112). 
The egg-shell appears soon as a close-fitting membrane. The 
germinal spot is enclosed in a remarkably tough membrane in 
all the species examined. Though the ovaries are connected 
with the skin by ducts the ova are dehisced by the breaking 
away of whole follicles, which then disintegrate. In the 
branchial region of B. minutus there is a general corre- 
spondence between these ducts and the gill-slits, as Spengel 
has observed. 
The testes are lobed masses placed in the same situation as 
the ovaries. The outer zone of each testicular follicle is made 
up of spherical ceils (figs. 108 and 109, a), which contain several 
(? eight) deeply-stained dots. These cells are young spermato- 
blasts, and the dots, which increase in size in the spermato- 
blasts of the inner zone, are the heads of spermatozoa which 
are finally set free into the central cavity. Here they are 
arranged in curious strings, which wave above parallel to each 
other in preserved specimens (fig. 108). The testes, when 
mature, break up in B. Kowalevskii as masses, but in B. 
Robinii they exude from the skin as a yellow slime. 
Mucus.—All the species secrete vast quantities of mucus 
when irritated. That of B. Robinii sets to form a mass of 
tough consistency, which collecting grains of sand forms a sort 
of tube. In this the animal can move slightly. The body of 
this species is very flat in the generative region, and is 
naturally folded up dorsalwards within the tube. The mucus 
of this form, which comes out after prolonged irritation, turns 
to a reddish-violet colour on exposure to the air, which is 
very characteristic. 
In B. Brooksii, Robinii, and salmoneus the sides of 
the body are produced dorsalwards into flaps which nearly 
meet in the branchial region, and thus cover the gill-slits and 
dorsal nervous system. 
