26 CLEAVAGE AND THE GERM LAYERS 



slowly than the upper small cells. At the blastula stage, the cavity is small, 

 and the cells of the vegetal pole are each many times larger than those at the 

 animal pole (4 and 5). The cleavage of the frog's ovum is thus complete but 



unequal. 



Cleavage in Reptiles and Birds.— The ova of these vertebrates contain a 

 large amount of yolk. There is very httle pure cytoplasm except at the animal 

 pole and here the nucleus is located (Fig. 3). When segmentatiou begins, the 

 first cleavage plane is vertical but the inert yolk does not cleave. The segmen- 

 tation is thus incomplete or meroUastic. In the hen's ovum the cytoplasm is 

 divided by successive vertical furrows into a mosaic of cells, which, as it increases 

 in size, forms a cap-like structure upon the surface of the yolk. These cells are 

 separated from the yolk beneath by horizontal cleavage furrows, and successive 

 horizontal cleavages give rise to several layers of cells. The space between 

 cells and yolk mass may be compared to the blastula cavity of Amphioxus and 

 the frog (Fig. 18). The cellular disc or cap is termed the germinal disc or blasto- 

 derm. The yolk mass, which forms the floor of the blastula cavity and the greater 

 part of the ovum, may be compared to the large yolk-laden cells at the vegetal 

 pole of the frog's blastula. The yolk mass never divides but is gradually used up 

 in supplying nutriment to the embryo which is developed from the cells of the 

 germinal disc. At the periphery of the germinal disc new cells constantly form 

 until they enclose the yolk. 



Cleavage in Mammals. — The ovum of all the higher mammals, like that 

 of man, is isolecithal and nearly microscopic in size. Its cleavage has been studied 

 in several mammals but the rabbit's ovum will serve as an example. The cleav- 

 age is complete and nearly equal (Fig. 17), a cluster of approximately uniform 

 cells being formed within the zona pellucida. This corresponds to the morula 

 stage of Amphioxus. Next an inner mass of cells is formed which is equivalent 

 to the germinal disc, or blastoderm, of the chick embryo (Fig. 17). The inner cell 

 mass is overgrown by an outer layer which is termed the trophcctoderm, because, 

 in mammals, it later supplies nutriment to the embryo from the uterine wall. 

 Fluid next appears between the outer layer and the inner cell mass, thereby sepa- 

 rating the two except at the animal pole. As the fluid increases in amount, a hol- 

 low vesicle results, its walls composed of the single-layered trophcctoderm except 

 where this is in contact with the inner cell mass. This stage is known as that of 

 the blastoder m ic vesicle. It is usually spherical or ovoid in form, as in tlie rabbit, 

 and probably this is the form of the human ovum at this stage. In the rabbit 

 the vesicle is 4.5 mm. long before it becomes embedded in the wall of the uterus. 



