3i8 A HISTORY OF FISHES 



as chromosomes, which form an important part of every cell 

 nucleus. 



The eggs of fishes present great diversity, not only in size 

 and shape, but in the manner in which they are protected 

 (Fig. 1 1 6). In the Cyclostomes there is a very striking difference 

 in the characters of the eggs in the two families. In the Lam- 

 preys [Petromyzonidae) they are minute, spherical, and enclosed 

 in delicate membranes, the average size being about one 

 millimetre (one-twenty-fifth of an inch) in diameter. In the 

 Hag-fishes [Myxinidae) on the other hand, they are large, 

 roughly spindle-shaped, and enclosed in tough horny capsules, 

 measuring up to thirty millimetres in length and ten millimetres 

 in width (Fig. ii6e). At the end of the capsule is a tuft of 

 horny processes, each of which ends in a tiny anchor-like hook, 



/feart 



Zkr 



/dcropylt'^ Germ. Yoik^ BlaiCodtrm. Taii^ //o^eAord yVoiocAord 



OVUM EMBRYO- a^OAYa -440AY3 - SHORTLY 6EFOR« 



^ HATCHING 



ySpermaibzoon. 



Fig. 115. 



Development of egg of Flounder (Flesus flesus). Much enlarged. (After 



Johnstone.) 



and in the middle of one of the tufts is the micropyle : the whole 

 of this end of the capsule is thrown oflf like a cap at the time of 

 hatching, thus allowing the young Hag-fish to make its escape. 

 The eggs are extruded one at a time, but are afterwards linked 

 together by means of the hook-like processes to form long 

 strings or bunches, usually attached to pieces of seaweed at 

 the bottom of the sea. Like all other large eggs, those of the 

 Hag-fish consist almost entirely of yolk, the essential portion 

 containing the nucleus being represented by a small hillock 

 at the end nearest to the micropyle. 



The Selachians may be oviparous or viviparous: that is to 

 say, they either produce eggs which are fertilised and left to 

 develop in the sea, or this development takes place in the oviduct 

 of the female, the young being finally born alive in an advanced 

 condition. The former is without doubt the more primitive 

 method, for in many, if not in all, viviparous Sharks and Rays 



