342 BLASTOPORE. 



means isolated, and should serve as a warning against laying too 

 much stress upon the frequency of the occurrence of invagination. 

 The great influence of the food-yolk upon the early development 

 might be illustrated by numerous examples, especially amongst 

 the Chordata (vide Chapter XI.). 



If the descendants of a form with a large amount of food-yolk 

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

 of formation 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 ancestors 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 processes occurs in the most primitive forms. If there 

 were any agreement amongst primitive forms as to the type of 

 their development this argument might have some weight. On 

 the whole, delamination is, no doubt, characteristic of many 

 primitive types, but the not infrequent occurrence of invagination 

 in both the Coelenterata and the Porifera the two groups which 

 would on all hands be admitted 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 



