Ch. VII] PRODUCTIOX OF ABNORMAL EMBRYOS 77 



of development the ridges are clearly seen to be the widely 

 separated halves of the medullary plate. Beneath each half 

 of the medullary plate there is a cross-section of the notochord, 

 and lietween the yolk-cells, in the centre of the section, and the 

 ectoderm covering the lower surface, there is a thick sheet of 

 cells representing the mesoderm. 



A longitudinal (sagittal) section of the embryo drawn in 

 Fig. 28, A, is shown in Fig. 29, C. The large exposure of yolk- 

 cells (Y) in the upper part of the figure is very conspicuous. 

 A deep and narrow depression, bounded for the most part by a 

 distinct layer of yolk-cells, is found near the anterior end. 

 This depression corresponds to the crescent-shaped opening 

 seen in surface view, and is supposed to correspond to a part 

 of the archenteron of the normal embryo. ^ Ectoderm covers 

 the lower (ventral) surface of this section, and at one point the 

 cells are thickened to form the adhesive glands of the larva. 

 At the posterior end of the embrj-o a small depression is pres- 

 ent, and, as later development shows, this corresponds to the 

 posterior portion of the archenteron of a normal embryo. 



HertA\ig found that if male and female frogs of certain 

 species be separated and kept apart for several weeks, and the 

 eggs then be artificially fertilized, an abnormal segmentation 

 follows, and, although many of the eggs die, among those that 

 live a large number show this condition of spina bifida. 



In 1893 I made a series of experiments attempting to pro- 

 duce artificially embrvos showing spina bifida, and found that 

 they could be made by two entirely different methods. If the 

 segmented egg, before the blastopore-lips appear, be placed in 

 water to which .6 per cent, of salt (XaCl) has been added, the 

 later development is modified. The dorsal lip of the blasto- 

 pore appears in its normal position but does not continue to 

 extend over the white hemisphere. The corners of the lips 

 gradually extend around the equator of the egg. A sharp 

 line or depression separates the black and white hemispheres, 

 and on the black side of the depression a circular ridge appears, 

 which marks the beginning of the medullary ring (Fig. 27, A). 



Similar embrvos may also be produced if tlie dorsal lip of 



1 Possibly it represents in part the liver-diverticulum. 



