WOLFFIAN DUCT AND BODY IN THE CHICK. 453 
But still, such as they are, it seems worth while to put them 
together, and to discuss the conclusions to which they seem to 
point. 
Mr. Balfour! has compared the embryonic record to an 
ancient manuscript in which many leaves are missing, many 
moved out of their proper order, and many spurious ones inter- 
polated by later hands. It is the duty of an embryologist to 
try to reconstruct the manuscript and see exactly what it contained 
when it was first written. In doing this he is aided by the fact that 
he has access to many copies of the manuscript, which have 
each been used and altered by very different people. He is thus 
able, by comparing the different copies, and by studying the 
characters, &c., of the people by whom they have been possessed, 
‘to arrive at a more correct idea as to what the original was like 
than if he had only one copy. 
In studying the various embryonic records we have we can 
pick out certain features common to all, and which may be 
assumed to have had their counterpart in the phylogenetic 
history. But the majority of features have been so altered that 
it is only possible to arrive at anything like a conclusion by 
taking into account the complicated conditions in which the 
animals have lived. 
Discussion of the preceding Facts. 
While the pronephros is characterised by a very similar struc- 
ture and development in all the animals in which it occurs, the 
mesonephros, though possessing in all animals a fairly similar 
adult structure, presents most remarkable differences in develop- 
ment in the different groups. While the mesonephros is uni- 
versally (few Teleostei excepted) present, the pronephros is only 
present in certain forms. Considering first the Ichthyopsida, it 
is at once seen that the presence or absence of a pronephros is 
correlated with another peculiarity. When the pronephros is 
present the egg contains a relatively small amount of food yolk, 
and the young undergo a considerable part of their development 
after leaving the egg; while, when the pronephros is absent, the 
egg contains a very bulky food yolk, and the young undergo 
far the greater part of their development within the egg (Hlas- 
mobranchii). 
Further, again considering the Ichthyopsida, we find that one 
method of development of the mesonephros is found in those 
animals with a pronephros, while the other method is found 
in those animals without a pronephros. Of the two methods of 
development of the mesonephros, while one (that found in 
} *Comp. Embryology.’ 
