578 REPRODUCTION OF ARGONAUT. 
its name, are females; and it now seems clear that the 
essential purpose of the shell is the protection, not of the 
animal (which is not in any way attached to it), but of the 
eggs. The male is of such comparatively insignificant size, 
_ that, not being provided with a shell, it was not recognised 
until recently as belonging to the same species. The sper- 
matic duct passes through one of its arms, which is extended 
into a long whip-like appendage ; and in this duct are found 
bundles of spermatozoids, all contained within one casing, 
which does not possess any self-moving power. At a certain 
epoch, this arm detaches itself entirely from the body, and 
moves freely through the water by means of the apparatus of 
nerves and muscles with which it is endowed, until it comes 
into contact with a female of its own species, whose eggs are 
fertilized by its contents which are then set free. In this de- 
tached condition, the arm was long since observed within the 
shell of the Argonaut, and was supposed to be a parasitic 
Worm ; subsequent inquiry showed it to possess, in its suckers 
and its nervo-muscular apparatus, the characteristic structure 
of the Cephalopod, and it was at first supposed to be itself the 
male, destitute (like the male of some Rotifera, § 750) of any 
nutritive apparatus. Its true history, as now elucidated, is 
one of the most curiously-exceptional phenomena in the whole 
of this department of physiology. 
754. Having at last arrived at the Vertebrated series, we 
shall take a general survey of the history of Development as 
presented to us in the case most familiar to every one, the 
formation of the Chick within the egg of the Bird; pointing 
out, as occasion arises, the principal points of: difference be- 
tween the mode in which the process is there carried-on, and 
the corresponding phenomena presented by other classes.— 
The ovum, as formed within the ovary, has neither “ white” 
nor “shell,” but consists of the yolk-bag and its contents 
alone. Under the influence of domestication, which affords 
a more constant supply of food and warmth than the Fowl 
would obtain in its natural condition, a much larger number 
of eggs are produced by the hen than she would produce in 
her wild state; so that, instead of laying four or five at a 
time, with long intervals between each deposit, she is conti- 
nually evolving them. An enormous quantity of “food- 
yolk” is prepared, in addition to the “germ-yolk ;” and thus 
the Bird’s egg comes to acquire a far larger size in proportion 
