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 " Urmand 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, Pfliiger, 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 



