THE OEIGTN OP THE METAZOA. 107 



The central protoplasm, which contains a few nuclei and a 

 great number of large and small vacuoles to which the nuclei 

 have at first no special relation, protrudes from the point at 

 which the cortical nuclei are absent^ as though to extend 

 itself in an amoeboid manner in search of food. This is the 

 solid gastrula stage (Plate IV, fig. 20). In it no cell outlines 

 are distinguishable, the whole embryo difi*ering only from, 

 say Vorticella, in its large size, and the presence of a 

 definitely arranged layer of nuclei round its periphery. 



From this stage the ccelo-gastrula is derived by the simple 

 process of the confluence of the larger central vacuoles to form 

 a single internal cavity, the establishment of the definite 

 opening of this cavity to the exterior, and the arrangement of 

 the central nuclei in the protoplasm lining it (Plate V, figs. 23, 

 24 b). Later the mesoderm appears. It is derived from 

 some of the nuclei already present, which increase in number 

 and arrange themselves in the protoplasm around some of the 

 vacuoles which thus early become specialised into another 

 organ, the coelom. 



MetschnikoQ', who has done such important service to 

 biology in drawing attention to the physiological importance 

 of amoeboid cells in the organism, has been one of the most 

 prominent advocates of the view that the formation of the 

 gastrula by invagination is a secondary process. He considers 

 that the animal of which the gastrula is the embryonic repro» 

 duction — if indeed it ever existed — was preceded by a form in 

 which the endoderm consisted of a vacuolated mass of proto- 

 plasm without any definite enteric cavity. So far most 

 embryologists of the present day are with him. But he goes 

 further ; he considers that this parenchymatous gastrula or 

 blastula^ — or as he calls it, Phagocytella — was preceded by 

 a hollow blastula-like Protozoon form from which it arose by 

 the migration inwards of certain of the cells of the wall of 

 the blastula. This suggestion as to the origin of the gastrula 



' I am not quite sure wLetber he considers that the cortical layer (kyuo- 

 blast) of his Phagocytella was interrupted at any point for the protrusion 

 of the central mass (phagocytoblast). 



