EMBRYOGENY 121 



to multiply in number, this disc gradually surrounds the yolk, which is 

 finally absorbed by the growing embryo. 



In centrolecithal egg cells the nucleus in the center of the cell divides 

 repeatedly, each of the daughter nuclei being surrounded by a little mass 

 of cytoplasm. As these nuclei increase in number they migrate toward 

 the periphery, accompanied by the bits of cytoplasm, and enter the super- 

 ficial cytoplasmic layer. Now division of the cytoplasm takes place by 

 planes which cut it at right angles to the surface, and this for a time leaves 

 each cell open toward the yolk in the center. A httle later the walls of 

 these cells become complete. Because a superficial layer of cells is in this 

 way formed around the yolk it is termed superficial cleavage (Fig. 49 D 

 and E). 



146. Steps in Embryogeny. — In the ideal embryogeny (Fig. 50) pre- 

 viously referred to as that of a homolecithal egg cell, cleavage may be 

 conceived as resulting in the development of a compact mass of cells 

 which, because of its general resemblance to the fruit of the mulberry tree, 

 has been called a morula. 



As the multiplication of cells continues, a cavity begins to form in the 

 mass. The embryo is then called a blastula. This cavity increases in 

 size until the blastula appears Hke a hollow rubber ball, the cells or 

 blastomeres forming the wall, which is now called the blastoderm. The 

 central cavity is variously known as the cleavage cavity, segmentation 

 cavity, or blastula cavity and also as the hlastocoel. The blastoderm in a 

 typical blastula is a single layer of cells, but in certain cases it is made 

 up of more than one layer. 



As cell division is still going on, the blastula tends to increase in size 

 with the increasing number of cells in the blastoderm, but these cells 

 differ in size and also in the rapidity of their multiplication. Those 

 toward the upper, or animal, pole are the smaller ones and are multi- 

 plying more rapidly, whereas those toward the other pole are larger and 

 are multiplying more slowly. This unequal growth causes an expansion 

 of the upper wall of the blastula and leads to an invagination of the lower 

 cells, the blastula thus becoming converted into a double-walled inverted 

 cup. As soon as this invagination begins, the embryo is termed a gas- 

 trula. As the gastrula develops further, the two walls come gradually 

 closer together until finally the cleavage cavity becomes entirely oblit- 

 erated. This process is called gastrulation, the cavity formed is known 

 as the archenteron, or primitive digestive cavity, and the opening into it 

 from the outside is termed the blastopore. The gastrula is thus made up 

 of two layers of cells ; the one forming the outer wall of the cup is called, 

 because of its position, the ectoderm, and the one within, forming the 

 lining, is known as the entoderm. 



Now a third layer of cells appears between the two others, being 

 developed in some cases from the ectoderm and in others from the 



