ONTOGENESIS 209 



dergo segmentation, and the primitive assemblage of cells 

 resulting from this localized cleavage appears as a cellu- 

 lar disc floating upon the surface of the yolk. This 

 germinal disc receives the name " blastoderm. 1 ' 



The segmentation first results in the formation of a 

 solid cellular mass which bears a partial resemblance 

 to a mulberry and is known as a morula. Every egg 

 passes through this stage. Soon, however, the morula 

 becomes changed by assumption of fluid or by vacuo- 

 lization of the inner cells, and a hollow sphere is formed, 

 the blastula, surrounded on all sides by a single layer of 

 blastomeres. 



The eggs of the invertebrates always form blastulae, 

 some of which are ciliated, free-swimming, and self- 

 sustaining larval forms. Such are called monoblastic 

 larva. They are typically centre-symmetric, the hollow 

 centre being known as the archicele or blastocele, the 

 cellular layer as the archiblast. The sponges have 

 larvae of this form. 



Holoblastic eggs of higher animals, having formed 

 a blastula, next undergo a peculiar invagination through 



SECTION OF GASTRULA. 



Archcnteron -*^ SECTION OF 



Epiblas 

 Hypoblast 



a 

 FIG. 85. a, Section of gastrula; b, section of planula. 



the ascent of the vegetative pole of the blastula layer, 

 until it comes into contact with the cells of the animal 

 pole. The invaginated blastula thus comes to resemble 

 a hollow ball one side of which has been pressed in 

 against the other. The result of the invagination is 

 that the embryo now consists of a double layer of cells, 

 surrounding a new cavity formed by the invagination, 

 U 



