CLEAVAGE 



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 blastoderm new cells constantly form until they enclose the yolk 

 (Fig- 1 8 C). 



Blastomere Blastocalt 



, -: ty. 



.'. .-_.>. .. ./ 



A B 



FIG. 16. Cleavage of the pigeon's ovum (after Blount). A, blastoderm in surface view; 



B, in vertical section. 



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 cleavage is complete and nearly equal (Pig. 17), a 

 cluster of approximately uniform cells being formed within th < e*' 

 pellucida. This corresponds to the morula stage of Amphioxus. Next 

 an inner mass of cells is formed that is equivalent to the germinal disc, or 

 blastoderm, of the chick embryo (Fig. 17). The inner cell mass is over- 

 grown by an outer layer which is termed the trophectoderm, 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 separating the two except at the animal pole. As the fluid in- 

 creases in amount, .a hollow vesicle results, its walls composed of the 

 single-layered trophectoderm except where this is in contact with the 

 inner cell mass. This stage is known as that of the blastodertnic 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 the 

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

 uterus. Among Ungulates (hoofed animals), the vesicle is greatly elon- 

 gated and attains a length of several centimeters, 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. 18). 

 In each case there is an inner cell mass of the germinal disc. The tro- 



