EXPERIMENTAL MORPHOGENESIS 73 



what has been ascertained about them in the different 

 classes of the animal kingdom. A full regulation of the 

 intimate structure of isolated blastomeres to a new whole, 

 has been proved to exist in the highest degree in the 

 eggs of all echinoderms, medusae, nemertines, Amphioxus, 

 fishes, and in one class of the Amphibia (the Urodela) ; it 

 is facultative only among the other class of Amphibia, the 

 Anura, and seems to be only partly developed or to be 

 wanting altogether among ctenophora, ascidia, annelids, 

 and mollusca. Peculiarities in the organisation of specific 

 parts of protoplasm have been proved to occur in more cases 

 than at first had been assumed ; they exist even in the 

 echinoderm egg, as experiments of the last few years have 

 shown ; even here a sort of specification exists at the 

 vegetative pole of the egg, though it is liable to a certain 

 kind of regulation ; the same is true in medusae, 

 nemertines, etc. ; but among molluscs, ascidians, and 

 annelids no regulation about the specific organisation of 

 the germ in cleavage has been found in any case. 



The differences in the degree of regulability of the 

 intimate germinal structure may easily be reduced to 

 simple differences in the physical consistency of their 

 protoplasm. 1 But all differences in specific organisation 

 must remain as they are for the present ; it will be one 

 of the aims of the future theory of development to trace 

 these differences also to a common source. 



That such an endeavour will probably be not without 

 success, is clear, I should think, from the mere fact that 



1 It deserves notice in this connection, that in some cases the protoplasm 

 of parts of a germ has been found to be more regulable in the earliest stages, 

 when it is very fluid, than later, when it is more stiff. 



