32 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



B. THE CLEAVAGE OF THE EGG AND THE FORMATION OF THE BLASTULA 



Development begins by the division of the egg into two, four, etc., cells until 

 a large number of cells has been produced. This process of division of the egg 

 is called cleavage, and the way in which it occurs depends on the amount of yolk 

 which the egg contains. It should be understood that the yolk is inert material 

 and that the process of development is carried out only by the living proto- 

 plasmic portions of the egg. 



1. Holobastic equal cleavage. In the case of isolecithal eggs the entire 

 egg divides and produces a number of approximately equal cells. Such cleavage 

 is said to be holoblastic and equal. The cells, as they increase in number, gradually 

 withdraw from the center and arrange themselves in a single layer on the surface, 

 thus producing a hollow ball of cells. This ball is called the blastula; its cavity 

 is known as the segmentation cavity or blastocoel. Such a blastula is produced 

 in the development of Amphioxus. Cleavage and formation of the blastula 

 in Amphioxus are illustrated in Figure $A. Similar figures will also be found in 

 the standard textbooks, as P and H, K, W, and H. Study also the models of 

 the cleavage of Amphioxus provided in the laboratory. 



2. Holobastic unequal cleavage. This type occurs in those telolecithal 

 eggs which contain a moderate amount of yolk. The half of the egg which con- 

 tains the majority of the yolk is called the vegetal hemisphere; that which 

 contains the majority of the protoplasm is the animal hemisphere. The early 

 cleavage planes are shifted toward the animal hemisphere, and further, the 

 cleavage processes are delayed in the vegetal hemisphere owing to the presence 

 of the inert yolk. In consequence of these two factors, the cells produced in the 

 animal hemisphere are smaller and more numerous than those of the vegetal 

 hemisphere, although the entire egg cleaves. Such cleavage is holoblastic but 

 unequal. The cells withdraw from the center, producing a blastula with a 

 somewhat reduced blastocoel and a wall several layers of cells thick. The cells 

 of the blastula are of unequal sizes grading from the smallest at the animal pole 

 to the largest at the vegetal pole. 



This type of development is characteristic of Amphibia. It is illustrated in 

 Figure ^B; similar figures are presented in various textbooks, as P and H, K, 

 and N. Study also the models of the cleavage of the amphibian egg, provided 

 in the laboratory. Then obtain a section through an amphibian egg in the 

 blastula stage and examine under the low power of the microscope. The blastula 

 is a hollow sphere whose wall is composed of two or three layers of cells. The 

 wall of the animal hemisphere is thin and consists of small cells; it is the future 

 dorsal side of the embryo. The wall of the vegetal hemisphere is much thicker 

 than that of the animal hemisphere and is composed of large cells, laden with yolk 

 and with indistinct boundaries; it is the future ventral side. The blastocoel is 

 smaller than in the blastula of Amphioxus, and is displaced dorsally, owing to 

 the thickness of the ventral wall. Draw, showing outlines only of the cells. 



