1897. NOTES AND COMMENTS. 5 



Alternation of Generations in Vertebrates. 



We liave had some difficulty in finding an appropriate heading 

 for this comment on a recent treatise by Mr. John Beard, entitled 

 " On Certain Problems of Vertebrate Embryology," and published at 

 Jena by Fischer. It deals incidentally with speculations as to the 

 ancestry of vertebrates, such as Dr. Gaskell's theory of arthropod origin 

 alluded to in our November number ; it discusses many particular 

 problems of embryology, such as the fate of the yolk and Mr. Beard's 

 own discovery of an ephemeral nervous system in vertebrate larvae ; 

 it has also an important bearing upon what is called the recapitulation 

 theory. But probably the central and most important idea in it is 

 the suggestion that all Metazoa, and in particular the Vertebrata, 

 have representatives of the two generations well known in plants, the 

 sporophyte or asexual generation, and the gametophyte or sexual 

 generation. Mr. Beard's position among vertebrate embryologists is 

 so assured, and this particular volume is so important a summing-up 

 of his work for the last eight years, that specialists certainly will study 

 the treatise for themselves. We shall attempt, therefore, in an 

 untechnical fashion to set forth the general lines of his brilliant theory. 



Two types of larvae are familiar to the naturalist, although in 

 many instances there is difficulty in determining under which type 

 particular larvae must be grouped. In one of these types the larva, 

 although it may attain a large size and be provided with special 

 organs, is no more than a bearer of the growing adult. The real 

 embryo appears as a small bud in some part of the large bearer, and 

 gradually grows until a point is reached at which it has become ipore 

 important than its nurse. The nurse or bearer then degenerates, and 

 frequently its broken down tissues are absorbed and digested by the 

 growing embryo. Such a larva, familiar among the echinoderms, may 

 be compared with the sporogonium of a moss, and, indeed, Mr. Beard 

 declares it to be, like the sporogonium, an alternating, sexless genera- 

 tion. On the other hand, there are larvae, like tadpoles, and probably 

 like those of at least most insects, which pass directly into the adult 

 form, changes no doubt taking place, but yet the general structure of 

 the larva being directly transformed into the general structure of the 

 adult. Such a larva may be compared to a young seedling, the leaves 

 and shape of which may differ from the adult but which directly 

 becomes the adult. Mr. Beard's great thesis is that in all vertebrate 

 development there is a stage easy to make out and directly com- 

 parable to the first kind of larva, that is to say, to an asexual, alternat- 

 ing generation. At a definite point of the development, different in 

 date from the beginning of the development in different vertebrates 

 but corresponding very closely in the differentiation of organs 

 attained, what he calls the critical stage is reached. The embryo 

 has become a fcetus ; the main lines of the future organism have been 

 laid down, and a sudden change in the mode of nutrition marks the 

 degeneration of those structures which correspond to the asexual 



