ZOOLOGY AND BOTANY, MICROSCOPY, ETC, 
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evolution or halmatogenesis, (3) on reproductive isolation (Kyeamechanie), 
as Eimer pointed out in 1874. 
Prof. Eimer gives concrete illustrations of liis position, and criticises 
Weismann’s. We cannot, however, enter into the discussion, interesting 
as it is ; the antithesis, roughly stated, is that to Eimer almost every- 
thing depends upon growth, to Weismann almost everything depends 
upon selection ; but it is perhaps more accurate to say that there is no 
antithesis, only a difficulty in appreciating the relative values of the 
two. 
Problems of Vertebrate Embryology.* — Dr. J. Beard has followed 
up his more concrete and technical papers with a general essay on some 
problems of Vertebrate embryology. The thread uniting its different 
parts is the idea of a substitution of organisms in Vertebrate ontogeny. 
He begins by stating the characters of “ the critical stage ” in 
Elasmobranch fishes. The embryo is beginning to show definite adult 
characters, and at this stage it annexes (into the gut) the external yolk- 
sac. In other Ichthyopsida, the critical stage is also found — nay more, all 
through up to Mammals. Thus the author points out most suggestively 
the correspondence of the critical stage in Scyllium with the birth-period 
in Didelphys , when, as before, a new mode of nutrition is initiated, and 
with the 15th day stage in the rabbit’s development, when the change 
to an allantoic placenta is realised. Valuable tables at the end of the 
essay make it easy to compare a variety of types at their critical stage. 
The author states von Baer’s laws, so often misstated, and would 
replace them by the following : — “ There is a stage [the critical stage] in 
the development of every Vertebrate embryo, during which, and only 
then, it resembles the embryo of any other Vertebrate in a corresponding 
stage in certain general features. But, while it thus agrees exactly with 
any other embryo in this stage in characters which are common to all 
Vertebrate animals, it differs from the embryo of any other class in certain 
special class-features, and also from any other embryo of the same class 
but of a different order in other and ordinal characters. Immediately 
before this stage is reached, it begins to put on generic and specific 
characters, and thus it then begins to differ from all other embryos in 
these.” 
“ The whole of this has its explanation in an antithetic alternation of 
generations as underlying the development ; for it is the stage at which 
the embryo first has acquired such an independence as will enable it to 
set about the task of suppressing the larval or asexual foundation, the 
phorozoon.” 
The Recapitulation Doctrine.f — Prof. F. Houssay has been prompted 
by Dr. Beard’s results and theories to discuss some aspects of ontogenetic 
recapitulation. While very appreciative of Beard’s work, he has some 
criticisms to make. The most important propositions are the following : 
— (1) The idea of “ a substitution of organisms ” (Beard) does not exclude 
a modified belief in ontogenetic recapitulation of phylogenetic stages ; 
(2) Beard has tended to confuse (in part, deliberately) the conception of 
alternation of generations on the one hand and of metamorphosis on the 
* ‘ On Certain Problems ofVertebrate Embryology,’ 8vo, Jena, 189G, vi. and 77 pp. 
t Anat. Anzeig., xiii. (1897) pp. 33-9. 
