172 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
planes of cleavage, and its effects can be traced even in the form of the 
embryo. 
If one of the first two blastomeres be completely destroyed, the sur- 
viving half forms a fairly normal embryo, with defects of slight import- 
ance; if the destruction be partial, division may still occur in the 
injured half, either in its own strength or with help from the uninjured 
half. But a destroyed half cannot be re-vitalized, nor does Roux’s post- 
generation occur. The development in an uninjured half is quite 
normal. No semi-gastrula lateralis, nor hemi-embryo lateralis, nor the 
like is ever formed when one of the first two blastomeres is destroyed ! 
The mosaic theory of development, the theory of organ-forming germinal 
areas, and the germ-plasm theory, are all contradicted by the facts. 
The ovum is a specifically organized “ Elementarorganismus,” which 
developes epigenetically, i. e. with gradual differentiation according to 
the mutual relations of the embryonic cells, for a normal embryo may 
develope from one of the first four blastomeres ; a disordered disposition 
of the nuclear materials has no effect ; by artificial devices the medul- 
lary plate, &c., may be made to arise from cell material quite different 
from that which is used in normal development. These and many other 
facts and conclusions does Hertwig bring together in formidable anta- 
gonism to Roux’s mosaic theory of development and Weismann’s 
theory of the germ-plasm. 
Breeding in and in.* — Dr. Ritzema Bos discusses the results of 
prolonged inter-breeding among nearly related animals. His experi- 
ments were made with the rat ( Mus decumanus). From seven of one 
family and an unrelated male which died after two crossings, he con- 
tinued breeding for six years without introducing fresh blood. There 
were about thirty generations. In 1887 the average number of a litter 
was 7J, in 1891 it suddenly sank to 4 T 7 ^, in 1892 to 3^. The rate of 
mortality increased enormously after about twenty generations, and 
the number of infertile pairings likewise increased. 
He concludes from his own and other experiments that continued 
breeding in and in lessens the reproductive capacity, may result in 
complete sterility, is associated with reduction of size, and perhaps 
causes greater predisposition to disease and malformations. 
Origin of Mammals.f — Dr. W. Haacke returns to the subject of a 
speculative paper which he published in 1887 on the origin of Mammals. 
He then associated the rise of Mammals with the Permian glacial epoch, 
but he is not so precise now. Their origin illustrates “ constitutional 
selection,” he says, which seems to mean that in the process of natural 
elimination those forms with less suitable constitutions would be most 
likely to go to the wall. We should hardly have thought that a new 
phrase was necessary to express the idea. 
He is pessimistic on the question of phylogeny. “We know nothing 
about the pedigree of Echinoderms, Crustaceans, Insects, Molluscs, and 
Annelids ; we know absolutely nothing about the origin of Vertebrates, 
and are not in a position to say anything about the ancestors of 
Amphibians and Reptiles, Birds and Mammals.” This is refreshingly 
* Biol. Centralbl., xiv. (1894) pp. 75-81. 
t Op. cit., xiii. (1893) pp. 719-32. 
