254 
F. M. BALFOUR. 
instances found coexisting in the same group renders it 
certain that there are not two independent phyla of the 
Metazoa, derived respectively from an invaginate and a 
delaminate gastrula.^ 
The four most important cases in which the two processes 
coexist are the Porifera, the Coelenterata, the Nemertea, and 
the Brachiopoda. In the cases of the Porifera and Coelen- 
terata, there do not appear to me to be any means of decid- 
ing which of these processes is derived from the other ; but 
in the Nemertea and the Brachiopoda the case is different. 
In all the types of Nemertea in which the development is 
relatively not abbreviated there is an invaginate gastrula, 
while in the types with a greatly abbreviated development 
there is a delaminate gastrula. It would seem to follow from 
this that a delaminate gastrula has here been a secondary 
result of an abbreviation in the development. In the Bra- 
chiopoda, again, the majority of types develop by a process 
of invagination, while Thecidium appears to develop by 
delamination ; here, also, the delaminate type would appear 
to be secondarily derived from the invaginate. 
If these considerations are justified, delamination must be 
in some instances secondarily derived from invagination ; 
and this fact is so far an argument in favour of the more 
primitive nature of invagination ; though it by no means 
follows that invagination contains a repetition of the steps 
by which the Metazoa were derived from the Protozoa. 
It does not, therefore, seem possible to decide conclusively 
in favour of either of these processes by a comparison of the 
cases where they occur in the same groups. 
The relative frequency of the two processes supplies us 
with another possible means for deciding between them ; and 
there is no doubt that here again the scale inclines towards 
invagination. It must, however, be borne in mind that the 
frequency of the process of invagination admits of another 
possible explanation. There is a continual tendency for 
^ It is not difficult to picture a possible derivation of delamination from 
invagination, while a eomparison of the formation of the inner layers 
(mesoblast and hypoblast) in Ascetta (amongst the sponges) and in the 
Echinoderraata shows a very simple way in which it is possible to conceive 
of a passage of delamination into invagination. In Ascetta the cells, which 
give rise to the mesoblast and hypoblast, are budded off from the inner wall 
of the blastosphere, especially at one point ; while in Eehinodermata 
(fig. 2) there is a small invaginated sac which gives rise to the hypoblast, 
while from the walls of this sac amoeboid cells are budded off whieh give rise 
to the mesoblast. If we suppose the hypoblast cells budded off at one point 
in Ascetta gradually to form an invaginated sac, while tlie mesoblast cells 
continued to be budded off as before, we should pass from the delaminate 
type of Ascetta to the invaginate type of an Echinodcrin. 
