DEVELOPMENT 303 



surfaces, and stick to one another as well as to fixed objects. 

 The spawn of the Smelt (Osmerus), which adheres to the gravel 

 bottom in estuaries, or to the piles of harbours and piers, is 

 peculiar in its manner of attachment. After extrusion, a portion 

 of the surrounding membrane of the egg breaks away and 

 becomes turned back, remaining attached to the egg at one 

 point, and it is by this piece of membrane that the egg is fixed. 

 In some of the Gobies (Gobiidae), Blennies (Blenniidae) and other 

 shore-dwelling fishes the eggs may be oval or pear-shaped, and 

 attached to rocks, stones, pieces of seaweed, shells and the 

 like by one end, and this end may be provided with a bunch of 

 adhesive filaments (Fig. ii6g). The eggs of the Skippers 

 (Scombresocidae) , Gar-fishes (Belonidae), and Flying-fishes {Exo- 

 coetidae) have sticky threads developed from opposite points 

 on their surfaces, which either serve to anchor them to foreign 

 objects or become entangled with those of other eggs of the 

 same species (Fig. 1 1 6f) . The eggs of nearly all fresh-water 

 fishes are adhesive, being attached to rocks or stones on the 

 river bed or to the leaves or stems of aquatic plants. The eggs 

 of the Perch (Perca) are held together by a membrane, and form 

 long, floating bands, attached at one end to water-weeds in 

 fairly still water. 



It will be impossible to follow in detail the manifold and 

 complex series of changes by which the fertilised egg is trans- 

 formed into a mature fish. Briefly, development starts by a 

 simple cleavage of the ovum into two halves, each of which 

 soon divides into two again, so that by a process of repeated 

 division, or segmentation, accompanied by regular growth, a 

 hollow ball of cells, the blastula, is formed, the walls of which 

 are made up of a single layer of cells. When the amount of yolk 

 present is very small and evenly distributed throughout the 

 egg, the cleavage occurs more or less uniformly all over, but 

 where the amount of food material is more or less abundant, 

 as is the case in most fishes, this may influence the segmentation 

 in one of two different ways. The yolk may tend to become 

 accumulated at one end of the ovum, the vegetative pole, and 

 the essential protoplasmic portion containing the nucleus to 

 be reduced to a relatively small cap situated at the opposite 

 end or animal pole. Thus, in the more primitive Bony Fishes 

 (Sturgeons, Bow-fins, Gar Pikes, Lung-fishes) the segmentation 

 starts at the animal pole, and always proceeds here at a faster 

 rate than at the \'egetative pole, so that when the blastula is 

 formed, the cells are much larger and fewer in number in the 



