Embryology. 



137 



of the evidences of phyletic (or ancestral) evolution, 

 which are so abundantly furnished by all the subse- 

 quent phases of ontogenetic (or individual) evolution. 

 The multicellular body which is formed by the 

 series of segmentations above described is at first a 

 sphere of cells (Fig. 40).- Soon, however, a watery 

 fluid gathers in the centre, and progressively pushes 

 the cells towards the circumference, until they there 



ect 



FIG. 41. Formation of the gastrula of Amp/noxtis. (After Kowalevsky. ) 

 A, wall of the ovum, composed of a single layer of cells ; B, a stage 

 in the process of gastrulation ; C, completion of the process; S, 

 original or segmentation cavity of ovum ; al, alimentary cavity of 

 gastrula ; ect, outer layer of cells ; ent, inner layer of cells ; b, orifice, 

 constituting the mouth in permanent forms. 



constitute a single layer. The ovum, therefore, is now 

 in the form of a hollow sphere containi g fluid, con- 

 fined within a continuous wall of cells (Fig. 41 A). The 

 next thing that happens is a pitting in of one portion of 

 the sphere (B). The pit becomes deeper and deeper, 

 until there is a complete invagination of this part of the 

 sphere the cellswhich constitute it being progressively 



