578 KEPRODUCTION 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 j 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 



