GERMIN.\L LAYERS OF THE EMBRYO. 
257 
yolk upon the early development might be illustrated by 
numerous examples, especially amongst the Chordata.^ 
If the descendants of a form with a large amount of food- 
yolk in its ova were to have 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 pro- 
bably 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 no special 
phylogenetic significance, and that the argument of 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 the most primitive types, but the not in- 
frequent occurrence of invagination in both the Coelen- 
terata 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 other- 
wise have. 
To sum up : in the present state of our knowledge there 
are no satisfactory data for deciding which of the two 
processes is the more primitive ; nor, considering the almost 
indisputable fact that both these processes have in many 
instances had a purely secondary origin, can any valid 
arguments be produced to show 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 independently of their 
occurrence in development. 
Though embryology does not at present furnish us with 
an answer to the question how the Metazoa became developed 
^ Vide r. M. Balfour, “ A comparison of the early stages iu the deve- 
lopment of Vertebrates,” ‘ Quart. Journ. Micr. Sci./ vol. 1875. 
