THE HERRING FAMILY. 411 



thus be compared with the vitelline membrane which occurs 

 in other fishes. Kupffer, on the other hand, maintained that 

 the herring was exceptional (though two membranes have been 

 described for the egg of the pike) in that the two layers must 

 be interpreted as two egg-membranes, the inner being the 

 commonly occurring vitelline membrane. On the whole it 

 would seem to be likely that the capsule of the egg of the 

 herring corresponds to that in other forms, but with a ten- 

 dency to greater differentiation of the laminae. 



The membrane is covered, immediately upon extrusion by 

 the female, by a layer of viscid material which causes the eggs 

 to adhere together or to any foreign body with which they may 

 come in contact. The viscid substance rapidly hardens on 

 exposure to the water and then presents a fibrous structure. 

 This occurrence of an adhesive material is not exceptional 

 amongst demersal eggs. The yolk is opaque, and is close to the 

 membrane, leaving a very small perivitelline space. Kupffer 

 found that the egg-elements could be distinguished under three 

 heads. Firstly, shining homogeneous globules which were 

 strongly refractive and varied in diameter from '008 to '02 mm. 

 These were massed together under the membrane and formed 

 the superficial layer of the yolk; he termed them yolk -granules. 

 Secondly, the large central mass of yolk, consisting of less 

 refractive bodies varying in size from '05 mm. to '08 mm. : these 

 he termed the yolk-globules. Lastly, distributed sparsely be- 

 tween these yolk-elements is a scanty viscous mass of proto- 

 plasm. According to this observer, the homogeneous structure 

 of yolk and yolk-protoplasm remains the same till fertilization, 

 and no blastodisc or collection of the protoplasm at one 

 pole is formed till spermatozoa have penetrated the egg-mem- 

 brane. Hoffmann, on the other hand, finds the herring no 

 exception to most other fishes in this respect also, and maintains 

 that the blastodisc is formed before fertilization. 



When this event (fertilization) has taken place the egg 

 shrinks away from its membrane, so that a space filled with 

 fluid is formed inside the capsule. At the same time, the 

 formative yolk, or protoplasm, separates from the nutritive yolk 

 as a continuous superficial layer surrounding the latter. Later 



