116 DEVELOP.MENT OF THE EliOG'S EGG [Ch. XI 



under tlie conditions of Roiix's experiment. The subsec.uent 

 postgeneration of the injured half of the egg has also been 

 studied by these authors. They confirm in every detail the 

 method of reorganization and postgeneration of tlie injured 

 half as described by Roux. The reorgamzing cells have migrated 

 Pom the wiinjured to the injured side, and there have caused the 

 protoplasm to break up into cells. The injured blastomere is 

 also overgrown directlg by the ectoderm of the uninjured and 

 developing side. In many of these embryos the right and 

 left side (one side has postgenerated) are separated from each 

 other by a protruding yolk-mass, forming spina-biiida embryos. 

 Ihe reorganization of tlie much changed mass of the injured 

 blastomere is brought about by being assimilated by the cells 

 that have migrated into that region, by the second and third 

 methods of reorganization described by Roux. When the mate- 

 rial of the mjure.l half is only incompletely reorganized, there 

 IS formed after postgeneration, a more or less pronounced spina 

 bihda. When the injured material is completely worked over 

 or reorganized and postgenerated, a perfect embryo mav be 

 formed. 



Schultze ('94, b, d) has made an interesting modification of 

 one of the experiments of Pfltiger and obtained most unex- 

 pected results. The eggs of Rana fusca removed from the 

 uterus were placed singly upon slides. On each slide had 

 been stuck two thin glass rods from 1.65 to 1.35 mm. in thick- 

 ness Between these rods, which are separated from each other 

 by the width of the slide, an egg is placed with tlie ^ohite pole 

 uppermost. The egg is then fertilized in this position. After 

 three minutes the spermatozoa may be supposed to have entered 

 and a glass cover is placed over the egg and brought down into 

 conflict with the two glass rods above-mentioned, and there 

 fixed with rubber rings. The egg is by this means slightly 

 coinpressed and held more or less firmly in position. Each 

 slide IS then turned over, i.e. through 180 degrees, so that the 

 dark pole of the compressed egg is brought upward. The 

 eggs now m the^iormal position are put into a dish of water, 

 to remain m this position until the first furrow has appeared or 

 even until it has passed through the egg. Then the slide and 



