ORIGIN OF THE (iElUHXAL LAYERS. 283 



its ova were to produce ova with but little food-yolk, the type of for- 

 mation of the germinal layers which would thereby result would be 

 by no means the same as that of the ancestors of the forms with much 

 food -yolk, but would probably be something very different, as in the 

 case of Mammalia. Yet amongst the countless generations of ances- 

 tors of most existing forms, such oscillations in the amount of the 

 food-yolk must have occurred in a large number of instances. 



The whole of the above considerations point towards the view that 

 the formation of the hypoblast by invagination, as it occurs in most 

 forms at the present day, can have in many instances no special 

 phylogenetic significance, and that the argument from frequency, in 

 favour of invagination as opposed to delamination, is not of prime 

 importance. 



A third possible method of deciding between delamination and 

 invagination is to be found in the consideration as to which of these 



O 



processes occurs in the most primitive forms. If there were any 

 agreement amongst primitive forms as to the type of their develop- 

 ment this argument might have some weight. On the whole, delami- 

 nation is, no doubt, characteristic of many primitive types, but the 

 not infrequent occurrence of invagination in both the Ccelenterata 

 and the Porifera the two groups which would on all hands be ad- 

 mitted to be amongst the most primitive deprives this argument of 

 much of the value it might otherwise have. 



To sum up considering the almost indisputable fact that both 

 the processes above dealt with have in many instances had a purely 

 secondary origin, no valid arguments can be produced to shew that 

 either of them reproduces the mode of passage between the Protozoa 

 and the ancestral two-layered Metazoa. These conclusions do not, 

 however, throw any doubt upon the fact that the gastrula, however 

 evolved, was a primitive form of the Metazoa; since this conclusion is 

 founded upon the actual existence of adult gastrula forms indepen- 

 dently of their occurrence in development. 



Though embryology does not at present furnish us with a definite 

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

 tozoa, it is nevertheless worth wliile 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 

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

 digestion ; wliile 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 tlie cells lining it, and the 

 mode of digestion generally characteristic of the Metazoa was thereby 



