1899] PRINCIPLES OF ANIMAL DEVELOPMENT 135 



be rightly concluded that fewer eggs have been laid, and that in 

 some form or other more incipient eggs have been used up in the 

 process of forming the functional one. 



This conclusion, or suspicion, brings us naturally to the main 

 points of our inquiry, i.e. to the modes in which food-yolk has been 

 obtained, and an egg-capsule for the reception of a single egg evolved. 



The simplest eggs are admittedly those, like the eggs of many 

 Echinoderms, which contain little or no food-yolk, possess only a single 

 membrane formed by the egg itself, and are laid singly, an egg-capsule 

 being entirely absent. 



It is always found that such simple eggs are laid in great numbers, 

 for few or none have been rendered sterile, or degraded to serve as 

 food to the others. 



Naturally in such cases the developing organism can attain no 

 great degree of complexity of structure before its original source of food- 

 supply, that contained in the egg itself, is used up. The organism must 

 then hatch out and seek food for itself from extraneous sources. Under 

 these conditions the resulting organism possesses at its birth the 

 simplest possible structure, that of a gastrula composed of two layers of 

 cells and with an aperture leading into the gut. 



Apparently there are two ways in which a further supply of 

 food might be bestowed on the developing egg. Some of its 

 fellows might be rendered sterile in the ovary, and there be used 

 up, as in liver-flukes, tapeworms, insects, etc., to increase the food- 

 supply of the remaining functional eggs. Or, a big batch of eggs being 

 deposited together within a simple structureless cocoon, as in earth- 

 worms, leeches, the lug-worm, and the whelk, some might be devoured 

 by others to form a reserve supply of food, as is the case in the whelk. 



Whether there is any connection between the two processes is a 

 difficult question to decide. It may be that they have been separately 

 evolved and that they have proceeded along parallel lines in their 

 subsequent history. On the other hand, in some cases, at any rate, it 

 is possible that the latter of the two processes was the original one, 

 and that in course of time it has passed into the former. 



If the indications are wanting, that the eggs of lampreys, ganoids, 

 and bony fishes ever passed through the latter condition of being 

 deposited in batches in simple cocoons, the evolution of a skate's egg 

 and purse is only intelligible on this supposition. 



Taking the latter process first of all, it will be noticed that it 

 ultimately leads to two results, — to the formation of a large-yolked egg 

 and to the evolution of a complicated egg-capsule for its reception. 



In its very beginning the formation of a simple cocoon entails the 

 co-operation of structures external to the ovary in the shape of genital 

 ducts. These are necessary in order that a cocoon, however simple in 

 form and structure, may be produced. It is the entire absence of any 

 such structure in connection with the eggs of lampreys, bony fishes, 



