THE EMBRYO 333 



the mulberry or morula stage. In the centre of the mass a 

 space is formed between the rounded inner surfaces of the cells, 

 and this grows larger as division proceeds. The free sur- 

 faces of the cells become flatter and they adhere more closely 

 to their neighbors. By this way we arrive at a stage called the 

 blastula, a hollow sphere, the wall of which is formed by a 

 single layer of cells. The cavity of the sphere is the blastula 

 cavity. 



713. The Gastrula. — By the next series of changes the blas- 

 tula is transformed into a gastrula. Cell division continues 

 and the embryo increases in size, but we will fix our attention 

 on another set of changes: First, the vegetative (yolk) pole 

 of the blastula becomes flattened, then concave, as seen from 

 the outside. This cavity deepens and thereby the blastula 

 cavity grows smaller, until it is finally obliterated, when the 

 inverted vegetative hemisphere comes in contact with the hem- 

 isphere of the animal pole. The embryo now has the form of 

 a cup, with double walls. This stage is the gastrula. The 

 new cavity which has been formed is the gastrula cavity 

 (archenteron), and its opening to the exterior is the gastrula 

 mouth. The two layers which form the wall of the gastrula 

 are the ectoderm and entoderm. After a time the embryo has 

 elongated, and by unequal growth its axis has been shifted so 

 that now the gastrula mouth, which has become very small, 

 lies at the posterior dorsal extremity. 



714. The Medullary Plate. — From now on several important 

 developmental processes occur simultaneously, but we will 

 trace them one by one. 



715. The dorsal surface of the elongated embryo becomes 

 flattened and the cells of the ectoderm along the median line 

 assume a columnar form, which results in a thickening of the 

 ectoderm. The medullary plate thus formed curls up along 

 its edges, forming a medullary groove. The edges of the groove 

 approach above and fuse to form the medullary tube. This 



