Embryology. 37 
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 
Fic. 41.—Formation of the gastrula of Amphioxus. (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 ef ovum; a/, alimentary cavity of 
gastiula; ec¢, outer layer of cells ; eze¢, inner layer of cells; 0, orifice, 
constituting the mouth in permanent forms. 
constitute a single layer. The ovum, therefore, is now 
in the form of a hollow sphere containing fluid, con- 
fined within a continuous wall of cells (Fig.41 A). The 
next thing that happens isa pitting in of one portion ot 
the sphere (B). The pit becomes deeper and deeper, 
until there is a complete invagination of this part of the 
sphere—the cells which constitute it being progressively 
