1 3 6 JOHN BEARD [ februa ry 



etc., which would appear to point to a different history of the yolk in 

 these cases. Reverting to the cocoon, a very simple structure, formed 

 by a genital duct, and containing a great number of small eggs with 

 little food-yolk, may be looked upon as the beginning of the process. 

 The lug- worm, Arenicola, furnishes an example. Larvae are developed 

 within the cocoon, and live there as long as they have food to consume. 

 If they then find no other nutriment within this structure they must 

 emerge, and seek for food elsewhere. 



Further life within the cocoon is only possible on one or other of 

 two conditions.' The substance of the cocoon itself, or a part of it, 

 i.e. all except its cortex, may be of a nutritive value. This is so in 

 certain leeches, for instance. Here the larvae, as soon as they reach 

 the gastrula condition, proceed to gorge themselves with the semi-fluid 

 " albumen " of the cocoon, and this food-supply suffices them until the 

 young leech is formed. 



But in some animals with simple cocoons the contents of the 

 cocoon apart from the eggs appear to possess no nutritive properties. 

 Undoubtedly it is to be looked upon as a secondary condition, when 

 the gelatinous " white " of the cocoon acquires a food value. 



A parallel to this is seen in certain eggs of vertebrate animals. 

 The egg-white of an Elasmobranch egg contains only the merest traces 

 of albumen (Johannes Muller), and is of no use as food. Whereas the 

 same egg-white in a bird's egg is largely albuminous and very 

 nutritive. 



But in the simple cocoon, although the substance of this structure 

 is sometimes of no value as a food, there may be unfertilised or un- 

 developed eggs, or abnormal or degenerated larvae, and these may serve 

 as food for the normal ones. Thus, in the second way, the " birth- 

 period " may be postponed. 



There are well-known cases among the Mollusca, Purpura and 

 Buccinum, in which this " cannibalism " is the normal course of events. 

 A few of the eggs in a cocoon develop quickly, and become gastrulae, 

 with large mouths and muscular gullets. These few very soon use up 

 their own food-supply, and then proceed to devour their fellows, which 

 have either not developed at all, or only gone a little way. If this 

 were carried to an extreme — and it is probable that such cases do 

 occur — it would end in there being only one developing organism left 

 in the cocoon, and this would be gorged with the food-yolk obtained 

 by the annexation of its fellows. 



The original cocoon by the solidification of its outer walls, its 

 centre remaining semi-fluid, might thus become an egg-case for a single 

 egg. This would be realised in fact if the eggs to be consumed, in- 

 stead of being deposited as separate entities within the cocoon, were 

 joined to or devoured by that single one which was destined to develop 

 while in the ovary. The well-known instances of Hydra and Tubu- 

 laria, with their ovarian " cannibalism," are obviously illustrative. 



