166 ZOOLOGY. 
The leech is hermaphroditic, while in certain allied forms 
(Histriobdella, etc.) the sexes are distinct. 
The eggs of leeches are laid in sacs, or, as in Clepsine, the 
fish-leech, are covered with a transparent fluid substance, 
which hardens and envelops the eggs. The Clepsine re- 
mains over the eggs to protect them until they hatch; and 
the young, after exclusion, fix themselves to the under side 
of the parent, and are thus borne about until they are fully 
developed and able to provide for themselves (Whitman*). 
The changes in the egg of Clepsine, after fertilization, are 
very complicated, and have been described by Whitman. 
The egg subdivides into a bilateral mass of cells called a 
blastula ; + a gastrula, and finally a “‘neurula” stage, charac- 
terized by the formation of a ‘“‘ primitive band” like that of 
insect embryos. Soon after attaining the latter stage the 
embryo hatches and attaches itself to its parent. The mouth 
is then formed, the nervous system{ arises from the ecto- 
derm, the segments are indicated, the original number being 
thirty-three, the segmental organs develop from the meso- 
derm at about the time of hatching, and about six days after 
the neurula leaves the egg the eyes become visible. The 
innermost germ-layer (endoderm) does not arise until eight 
days after hatching, and by this time the digestive tract is 
perfected ; the muscular walls of the alimentary canal being 
derived from the mesoderm. 
*The Embryology of Clepsine. By C. O, Whitman. Quarterly 
Journal of Microscopical Science. July, 1878. 
+ Whitman states that a morula,as defined by Haeckel, does not 
occur in the developmental history of Clepsine, and he states that when 
the cleavage process of the egg has been carefully studied it has been 
found to result in the production of a bilateral germ or blastula, and 
not a morula, ‘‘‘A solid sphere of indifferent cells’ is, to say the 
least, a very improbable form, so improbable that its existence may be 
held questionable until established by positive evidence. The doubt 
is all the more justifiable, as more careful investigation has in many 
cases already shown that the so-called mulberry stage is not a morula, 
but a blastula or even a gastrula.” (Whitman.) 
¢ There is originally a pair of ganglia in each of the thirty-three 
segments; four of these are consolidated into the subcesophageal gan- 
glia, eight in the ganglia of the disk, and four in the terminal ganglia 
of the body. (Whitman.) 
