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 u 

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

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

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

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

 plasmic portions of the egg. 



i. Holobastic equal cleavage. — In the case of isolecithal eggs the enl 

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

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

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

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

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

 in the development of Ampkioxus. Cleavage and formation of the blasti 

 in Amphioxus are illustrated in Figure 4A. Similar figures will also be found 

 K, page 9; W, page 59; P and H, page 56; H, page 142. Study also the mod 

 of the cleavage of Amphioxus provided in the laboratory. 



2. Holobastic unequal cleavage. — This type occurs in those telolecitl 

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

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

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

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

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

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

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

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

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

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

 of the blastula are of unequal sizes grading from the smallest at. the animal pc 

 to the largest at the vegetal pole. 



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

 Figure 4B; similar figures are presented in P and H, page 289; K, page 7; ] 

 page 206. Study also the models of the cleavage of the amphibian egg, P rovid 

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

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

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

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

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

 than that of the ammal hemisphere and is composed of large cells, laden with yo 

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

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

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



