256 
F. M. BALFOUR. 
last heading vary considerably in the character of the blas- 
topore. In some cases the fact of its not coinciding either 
with the mouth or anus appears to be due simply to the 
presence of a large amount of food-yolk. The cases of the 
Cephalopoda, of Euaxes, and perhaps of Clepsine and 
Bonellia, are to be explained in this way : in the case of 
all these forms, except Bonellia, the blastopore has the form 
of an elongated slit along the ventral surface. This type 
of blastopore is characteristic of the Mollusca generally, 
of the Polyzoa, of the Nematelminthes, and very possibly 
of the Chsetopoda and Discophora. In the Brachio- 
poda and the Chsetognatha (fig. 12 b) the blastopore is 
situated, so far as can be determined, behind the future 
anus. In many Decapoda, amongst the Crustacea, the blas- 
topore is placed behind, but not far from, the anus. In 
the Chordata it is also placed posteriorly, and, remarkably 
enough, remains, in a large number of forms, for some time 
in connection with the neural tube by a neurenteric 
canal. 
The great variations in the character of the gastrula, in- 
dicated in the above summary, go far to show that if 
the gastrulee, as we find them in most types, have any an- 
cestral characters, these characters can only be very general 
ones. This may best be shown by the consideration of a few 
striking instances. The blastopore in Mollusca has an elon- 
gated slit-like form, extending along the ventral surface from 
the mouth to the anusj in Echinodermata it is a narrow 
pore, remaining as the anus. In most Chsetopoda it is a 
pore remaining as the mouth, but in some as the anus. In 
Chordata it is a posteriorly-placed pore, opening into the 
neural canal. 
• It is clearly out of the question to explain these differ- 
ences in connection with the characters of ancestral forms. 
They can only be accounted for as secondary adaptations for 
the convenience of development. 
The epibolic gastrula of Mammalia is a still more striking 
case of a secondary embryonic process, and is not directly 
derived from the gastrula of the lower Chordata. It pro- 
bably originated in connection with the loss of food-yolk 
which took place on the establishment of a placental nutri- 
tion for the foetus. The epibolic gastrula of the Scorpion, 
of Isopods, and of other Arthropoda, seems also to be a derived 
gastrula. These instances of secondary gastrulae are very 
probably by no 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- 
