CHAPTER X 



EMBRYOLOGY AND GROWTH OF FISHES 



EGMENTATION of the Egg.— The egg of the fish de- 

 velops only after fertilization (amphimixis). This 

 process is the union of its nuclear substance with 

 that of the sperm-cell from the male, each cell carrying . its 

 equal share in the function of heredity. When this process 

 takes place the egg is ready to begin its segmentation. The 

 eggs of all fishes are single cells containing more or less food- 

 yolk. The presence of this food-yolk affects the manner of 

 segmentation in general, those eggs having the least amount 

 of food-yolk developing most typically. The simplest of all fish- 

 like vertebrates, the lancelet (Branchiostoma) has very small 

 eggs, and in their early development it passes through stages 

 that are typical for all many-celled animals. The first stage in 

 development is the simple splitting of the egg into two halves. 

 These two daughter cells next divide so that there are four cells ; 

 each of these divides, and this division is repeated until a great 

 number of cells is produced. The phenomenon of repeated di- 

 vision of the germ-cell is called cleavage, and this cleavage is the 

 first stage of development in the case of all many-celled animals. 

 Instead of forming a solid mass the cells arrange themselves in 

 such a way as to form a hollow ball, the wall being a layer one 

 cell thick. The included cavity is called the segmentation 

 cavity, and the whole structure is known as a blastula. This 

 stage also is common to all the many-celled animals. The 

 next stage is the conversion of the blastula into a double- 

 walled cup, known as a gastrula by the pushing in of one 

 side. All the cells of the blastula are very small, but those 

 on one side are somewhat larger than those of the other, 

 and here the wall first flattens and then bends in until 

 finally the larger cells come into contact with the smaller and 

 the segmentation cavity is entirely obliterated. There is now 



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