IN THE EMBRYOGESY OF THE HIGHER ANIMALS. 163 



press the conviction that the one is but a farther advanced 

 stage of the other. Here the majority of naturalists regard 

 the process as a modification of alternation, differing only 

 from that of the Trematoda in the gemmation developing 

 but a single offset from the primary zooid, instead of 

 a numerous brood, each individual of which is in turn 

 equally prolific.* 



Yet in this same class there are other species (Echin- 

 aster and Holothuria) in which the organization of the 

 original germ-mass is so little advanced, and that of the 

 typical Echinoderm is so confounded with it, that there is 

 positively less distinction between the two, than exists in 

 the evolution of the higher animals, between the so-called 

 mulberry body, and the proper embryonic organization which 

 subsequently originates from it. Dr. Carpenter, indeed, 

 points out differences in the embryogeny of the Vertebrata, 

 in some degree analogous to those just referred to in the 

 development of the Echinodermata, observing that the um- 

 bilical vesicle, which is cast off from the foetus of the Mam- 

 mal like the Bipinnaria of the Starfish is gradually 

 taken like the Pluteus of the Echinus into the body of 

 the Bird ; while he finds in the mode of evolution of the 

 Batrachian Reptiles, the whole of whose vitelline mass goes 

 at once to be converted into the embryo, a representation 

 of the type of development occurring in Echmaster.-^ 



* There may be added one other point of difference namely, that in 

 all cases, even where there is the greatest extent of change, certain parts 

 of the original structure particularly the alimentary canal are incorpo- 

 rated into the new growth. 



t Principles of Comparative Physiology, 555. A practical difficulty 

 which results from this diversity in the development of the Echinoder- 

 mata, is that it becomes impossible to apply the same terms with perfect 

 accuracy to all the cases, for while there may be little reason to object to 

 Auricularia being spoken of as the larva of Holothuria, we cannot so 

 well apply the same name to the Bipinnaria of the Starfish, which, after 

 it has parted from its derivative Echinoderm, continues for some time its 



