CELL-DIVISION 



medium show the filaments very beautifully as the figure 

 taken from Herbst so clearly shows^ (Fig. 36). Herbst 

 himself thought that he had altered the structure of the 

 hyaline plasma-layer, changing its homogeneous structure 

 to a collection of radial threads whereas actually by the 

 treatment with calcium-free sea-water, he only removed 

 the covering membrane of the threads. This change in 

 the eggs' environment prevents the formation of a new 

 membrane which takes place when the eggs are returned to 

 normal sea-water. The so-called removal of the hyaline 

 plasma-layer by microdissection is also a removal only of 

 the covering membrane. 



Goldschmidt and Popoff advanced the idea that the ecto- 

 plasmic filaments on the sea-urchins' egg are due to astral 

 radiations from the nucleus. But there are no astral radia- 

 tions in an unfertilized egg momentarily subjected to the 

 action of strong hypertonic sea-water; nor are any present 

 in an egg fifty seconds after insemination. In both cases, 

 filaments can be observed. Fertilized sea-urchins' eggs 

 shrink slightly when the astral rays are first most extensive 

 so that the eggs' surface appears somewhat crenated because 

 the filaments stand out strongly. It is this crenation that 

 these authors observed. 



Gray and Goldschmidt and Popoff have derived theories 

 of cell-division from their observations on the behavior of 

 the hyaline plasma-layer. Since these observations are 

 incorrect, the interpretation given them is unwarranted. 

 This is thus another example of the danger of applying 

 physico-chemical notions to a biological process in the 

 absence of knowledge of the underlying structural condition. 



During a cleavage-cycle of sea-urchins' eggs the ecto- 

 plasm undergoes changes. Until the stage- when the 



^ Herbst, i8gg. 



~ See Ziegler, 1^04. Compare Ziegler, iSgS, on egg of Bero'e. 



2S1 



