34 ADAM SEDGWICK. 



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 %^^ 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 e^g (Elas- 

 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 iu 



' ' Comp. Embryology.' 



