THE Ml^SI-X'iM. 



57 



a big batch of eggs being deposited to- 

 gether 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 in the case 

 of the whelk. 



Whether there is any connection be- 

 tween the two processes is a difficult 

 question to decide. It may be that 

 they have been separately evolved and 

 that they have proceeded along paral- 

 lel! 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 evolu- 

 tion 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 the evolution 

 of a complicated egg-capsule for its 

 reception. 



In its very beginning the formation 

 of a simple cocoon entails the co-oper- 

 ation 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 such structure in con- 

 nection with the eggs of lampreys, 

 bony fishes, etc., which would appear 

 to point to a different history of the 

 yolk in these cases. Reverting to the 

 cocoon, a very simple structure, form- 

 ed 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 ex- 

 am nle. 



Larvae are developed within the co- 

 coon and live there as long as they 

 have food to consume. If they then 



find no other nutriment within this 

 structure they must erperge and seek 

 for food elsewhere. 



Further life within the cocoon is 

 only possible on one or other of two 

 conditions. The substance of the co- 

 coon itself, or a part of it, i. e. , all 

 except its contex, 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 un- 

 til the young leech is formed. 



But in some animals with simple co- 

 coons 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 

 conaition when the gelatinous "white" 

 of the cocoon acquires a food value. 



A parallell to this is seen in certain 

 eggs of vertebrate animals. The egg- 

 white of an Elasmobranch egg con- 

 tains only the merest traces of albumen 

 Johnnes 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 some- 

 times of no value as food, there may 

 be unfertili/;ed or undeveloped 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 mullusca, Purpura and Buccinum, 

 in which this "cannibalism" is the nor- 

 mal 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 fel- 

 lows, 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 oc 

 cur — it would end in their being only 

 one developing organism left in the co- 



'-^B'^^«9n^!C*K«> 



