IV. GENERAL FEATURES OF CHORDATE DEVELOPMENT 



Since it is impossible to understand the comparative anatomy of vertebrates, 

 which forms the main object of study of this course, without some knowledge 

 of the way in which a vertebrate develops, it is necessary that the student learn 

 some of the elementary facts about vertebrate development. Every student 

 must read either K, pages 7-16, or W, pages 60-71. He will be expected to 

 know the contents of these pages thoroughly and to be able to answer at all 

 times questions relating to them. 



A. THE CHORDATE EGG 



The manner of development of the chordate egg is dependent upon the 

 amount of food material, or yolk, which it contains. On this basis chordate 

 eggs are classified as follows: (i) Isolecithal eggs, with little yolk; Amphioxus 

 and mammals. (2) Telolecithal eggs with total cleavage. There is a moderate 



animal pole \ germinal disk , 



, vegetal pole 



,yolk 



FIG. 3. Three types of chordate eggs. The black coloring represents the yolk. A, isolecithal 

 egg with small and evenly distributed yolk particles. B, telolecithal egg with total unequal cleavage, 

 with the yolk more abundant in the vegetal than in the animal half. C, telolecithal egg with mero- 

 blastic cleavage, consisting completely of yolk except for the small germinal disk of protoplasm at one 

 pole. The size of the germinal disk in the figure is greatly exaggerated with respect to the size of the 

 yolk. 



amount of yolk which accumulates in one-half of the egg and retards its develop- 

 ment; Amphibia, cyclostomes, and ganoid fishes. (3) Telolecithal eggs with 

 meroblastic cleavage. There is an enormous amount of yolk, and the protoplasm 

 is reduced to a small disk which floats on the surface of the yolk. This spot is 

 called the germinal disk. Such eggs are characteristic of birds, reptiles, and 

 teleosts. The true egg in the case of birds is the yolk, the white being merely 

 a nutritive envelope. 



These three types of eggs are illustrated diagrammatically in Figure 3. 

 (See further, H, pp. 141-48.) 



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