ORIGIN OF THE GERiMINAL LAYERS. 



343 



existence of adult gastrula forms independently of their occur- 

 rence in development. 



Though embryology does not at present furnish us with a definite 

 answer to the question how the Metazoa became developed from the Pro- 

 tozoa, it is nevertheless worth while reviewing some of the processes by 

 which this can be conceived to have occurred. 



On purely a priori grounds there is in my opinion more to be said for 

 invagination than for any other view. 



On this view we may suppose that the colony of Protozoa in the course 

 of conversion into Metazoa had the form of a blastosphere ; and that at 

 one pole of this a depression appeared. The cells lining this depression we 



Fig. 2- 



FIG. 205. DIAGRAM SHEWING THE FORMATION OF A GASTRULA BY 

 DELAMINATION. (From Lankester.) 



Fig. i, ovum; fig. i, stage in segmentation ; fig. 3, commencement of delamination 

 after the appearance of a central cavity; fig. 4, delamination completed, mouth form- 

 ing at M. In figs, i, 2, and 3, EC. is ectoplasm, and En. is endoplasm. In fig. 4, 

 EC. is epiblast, and En. hypoblast. E. and F. food particles. 



may suppose to have been amceboid, and to have carried on the work of 

 digestion ; while the remaining cells were probably ciliated. The digestion 

 may be supposed to have been at first carried on in the interior of the cells, 

 as in the Protozoa ; but, as the depression became deeper (in order to 

 increase the area of nutritive cells and to retain the food) a digestive 

 secretion probably became poured out from the cells lining it, and the mode 

 of digestion generally characteristic of the Metazoa was thereby inaugurated. 

 It may be noted that an intracellular protozoon type of digestion persists in 

 the Porifera, and appears also to occur in many Ccelenterata, Turbellaria, 



