ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
509 
monstrosities. Even a solution of *1 per cent, lias a marked influence. 
The ovum is unequally affected, the vegetative half being more inhibited 
and injured than the animal half, probably because less protoplasmic. 
The process of gastrulation is modified, approaching that in mero- 
blastic ova. There is a disproportionate increase of the dorsal organs, 
a longitudinal growth of the embryo, at the expense of the region 
adjacent to the blastopore ring, the cellular material being moved 
forwards and towards the median plane. Hertwig has observed three 
modes of gastrulation in frog-ova, and is convinced that the disposal of 
cell-material is determined in the course of the developmental process, 
according to the various conditions, and that the mosaic theory is thereby 
proved false. Another effect of the chemical stimulus of the salt is 
anencephaly and associated hemicrania, the cerebral plate being at an 
early stage inhibited from closing into a tube. 
It is hardly necessary to say that Hertwig maintains that external 
conditions have a direct influence, and are more than mere stimuli. 
External and internal conditions combine to give the result. He 
criticises Weismann’s position, and argues for Epigenesis. 
Grafting Amphibia.* — Prof. G. Born has been making some experi- 
ments on young tadpoles of various Amphibia. He was led to his 
experiments by the observation that when a tadpole was cut into two 
pieces the pieces might unite again if kept close together. He used 
larvae that were ready to escape from the egg jelly or those that had just 
escaped. When the tail ends of two tadpoles of the edible frog are 
placed with the cut ends together, they unite in 24 hours quite com- 
pletely, so that there is little external evidence of the line of fusion. 
These joined tails live for eight days and increase in length ; they then 
degenerate and become dropsical. The anterior ends of two larvae may 
be made to unite. This succeeds more readily with the younger stages. 
Complex unions of two larvae may result when the cuts are not quite 
complete and the two pieces of each remain connected by a slender 
bridge of tissue. The anterior part of one larva may be united to the 
posterior part of another individual. Two frog larvae may be easily 
united belly to belly so that a true twin is formed. It is possible to 
unite larvae of different genera, and even of different families. 
What is a Germinal Layer ? j — Dr. E. Braem criticises the different 
conceptions which prevail as to the nature of a germinal layer, and 
traces the history of opinion in regard to this question. Pander (1817) 
seems to have had the physiological differences of the layers most in 
view ; to him they were distinguished by the quality of the organs to 
which they gave rise. Von Baer (1828) also found the essential point 
of distinction in the nature of the organs which the layers form. Bathke 
(1829) also found the “idea” of a germinal layer in its function, in 
what it formed. So when Huxley (1849) compared the outer and inner 
layer of the Coelenterate body with the germinal layers of higher 
animals, the parallel rested on physiological relations. Remak (1851-5) 
distinguished sensory, motor-germinative, and trophic layers, and still 
kept to the physiological standpoint. Allman (1853) introduced the 
* JB. Schles. Ges., 1894. See Amer. Natural., xxix. (1895) pp. 590-2. 
t Biol. Centralbl., xv. (1895) pp. 427-43 (2 figs.), 466-76, 491-506. 
