84 



CHAPTER VI. 



EMBEYOLOGY. 



The ripe ovum of Solea vulgaris after it lias been removed from tlie body of tlie fish 

 and is floating in sea water, and when it has not been fertilised, has the following 

 structure: The whole ovum is a spherical transparent body measuring 1"47 to \b\ 

 mm. ("OS to '06 inch) in diameter, the size of different eggs varying within these 

 limits. The ovum consists of a definite thin transparent membrane surrounding a 

 solid mass ; the former may be called the vitelline membrane, the latter the ovum in a 

 stricter sense of the word. The whole of the egg with its envelope corresponds only 

 to the yolk of a hen's egg, there is nothing about it which represents the " white " or 

 the shell of the latter. The yolk of a hen's egg is surrounded by a thin membrane of 

 its own which corresponds to the vitelline membrane of the sole's egg. The ovum 

 proper within the vitelline membrane consists of two different parts : a smaller 

 somewhat protuberant portion which is more granular and less transparent, and a 

 more transparent larger portion. The former is the germ (blastodisc), and consists of 

 living organic matter known as protoplasm. Tliis germ is actually alive, and it 

 exhibits spontaneous movements and changes which gradually lead to the formation of 

 the young fish, while the larger portion is the yolk, and is simply a supply of very 

 nutritious food on which the germ lives and by means of which it grows. The yolk is 

 in fact gradually absorbed by the embryo during its development. In consequence of 

 the protuberance of the germ, the vitelline membrane being stiff and spherical, there is 

 a space between the membrane and the surface of the ovum all round the junction of 

 the germ with the yolk : this is the perivitelline space. Immediately beneath the germ 

 the yolk is divided up into large separate masses of a cubical shape : these masses 

 form a single layer, the rest of the yolk being undivided. The yolk is more or less 

 liquid, and it is confined within a delicate pellicle of protoplasm which extends from 

 the edge of the germ all round the yolk. Thus the yolk is really contained within the 

 protoplasmic germ. In fact the germ is the essential part of the ovum ; some animals 

 produce ova without either membrane or yolk : the yolk is to be regarded as an 

 accumulation of food material within the protoplasm of a reproductive "cell" or plastid. 

 On the surface of the yolk, immediately beneath its protoplasmic pellicle, are several 

 of minute oil globules. On account of their refracting power these appear 



