36 SEGMENTATION OF THE FERTILIZED OVUM 



and here the nucleus is located (Fig. 2). When segmentation begins, the first 

 cleavage plane is vertical but the yolk, being lifeless matter, does not cleave. 

 The segmentation is thus incomplete or meroblastic. In the hen's ovum the cy- 

 toplasm 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 Am- 

 phioxus and the frog (Fig. 17). The cellular disc or cap is termed the germinal 

 area or disc. 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 gradu- 

 ally used up in supplying nutriment to the embryo which is developed from the 

 cells of the germinal area. Round the periphery of the germinal area new cells 

 constantly form until they surround the yolk. 



The Ovum of the Rabbit. The ovum of all the higher mammals, like that 

 of man, is microscopic in size and nearly alecithal (no yolk). Its segmentation 

 has been studied in several mammals but we shall take the rabbit's ovum as an 

 example. The cleavage is complete and nearly equal (Fig. 16), a cluster of nearly 

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

 morula stage of Amphioxus. Next an inner mass of cells is formed which corre- 

 sponds to the germinal area, or blastoderm, of the chick embryo (Fig. 16). The 

 inner cell mass is overgrown by an outer layer which we term the troph-ectoderm 

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

 Between the outer layer and the inner cell mass fluid next appears, separating 

 them except at the animal pole. As the fluid increases in amount, a hollow vesicle 

 results, its wall composed of the single-layered troph-ectoderm except where this 

 is in contact with the inner cell mass. This stage is known as the germinal or 

 Uastodermic vesicle. It is usually spherical or ovoid in form, as in the rabbit, and 

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

 of macroscopic size before it becomes embedded. Among Ungulates (hoofed 

 animals) the vesicle is greatly elongated and attains a length of several centi- 

 meters, as in the pig. 



If we compare the mammalian blastodermic vesicle with the blastula stages 

 of Amphioxus, the frog and the bird, it will be seen that it is to be homologized 

 with the bird's blastula, not with that of Amphioxus (Fig. 17). In each case 

 there is an inner cell mass of the germinal area. The troph-ectoderm of the 



