APRIL, 1893. EXPERIMENTAL EMBRYOLOGY. 295 
methods. ‘‘ Whatever the disturbing agent may be which is em- 
ployed, the failures of development are generally of the same nature 
—a consideration which, if true, as I believe, points to a much 
deeper explanation of the variations and malformations produced 
than that which attributes them to the actual deprivation of air, or 
whatever the means may be which is used.” ‘I think there is now 
good evidence to prove that these disturbing agents act, at least in 
the great majority of cases, on that part of the developing organisa- 
tion which is concerned with the formation of the vascular system of 
the embryo.” 
Professor O. Hertwig has recently (1892) published an elaborate 
essay, entitled ‘‘ Urmund und Spina bifida,” based on a study of abnor- 
malities which occur in the development of frog ova in consequence 
of over-ripeness and polyspermy. The segmentation may be strangely 
irregular, large parts of the yolk may remain undivided, and the 
formation of the gastrula may be so disturbed that the blastopore 
or the primitive groove remains partially or wholly unclosed. He 
believes that the malformations which occur in the frog embryos in 
consequence of the imperfect closure of the ‘‘ Urmund”’ are analogous 
to the Terata mesodidyma and katadidyma in Teleostean fishes and 
to Spina bifida in higher vertebrates. In the over-fertilised egg 
opposing factors conflict ; on the one hand, there are forces tending 
to development and incited by the fertilisation, on the other hand, 
there are forces of an inhibiting and disturbing nature, due to the 
injury the ovum has sustained by over-maturation, or by other 
abnormal conditions operative before fertilisation. 
II. We shall now notice some results of the important work of Dr. 
W. Roux. It is well-known that, in a number of types, the first three 
planes of segmentation in the dividing ovum correspond to the three 
chief planes dividing the bilaterally symmetrical adult into right and 
left, rostral and caudal, dorsal and ventral regions. In Amphibians 
and Ascidians, the first plane of segmentation in the ovum normally 
corresponds to the median plane of the embryo. This was shown for 
Amphibians by Newport, Pfliger, and Roux, and for Ascidians by 
E.van Beneden and Ch. Julin. Of the first two cells into which the 
egg of a frog develops, one has in it the material for forming the right 
half of the body, the other has in it the material for forming the left 
half of the body. It is a separate question whether each cell has, by 
itself, the power of forming half a body. 
For a time, there was considerable difference of opinion as to the 
possible influence of gravity in determining the first segmentation 
planes. Pfliiger (1883) maintained that gravity determined the 
arrangement of the molecules, determining, for instance, in the un- 
segmented ovum the position of the future nervous system or dorsal* 
line; but Roux showed that when frog ova developed on a slowly 
rotating vertical wheel, so that the direction of the force of gravity was 
continually being changed, the segmentation remained normal. In 
