528 CELL DIVISION IN EGGS OF CREPIDULA. 



so, though development had been much retarded. In twelve other experiments 

 (1010-1022), in which the tubes were carefully stoppered after the eggs were 

 introduced, development was not only retarded but, in many cases, completely 

 stopped. In these eggs, nuclei and nucleoli are larger than usual, which may be 

 explained by the duration of the resting stage (interkinesis). The chromatin 

 within the resting nucleus is usually clumped into a nucleolus-like mass and the 

 spheres have lost their definite outlines and consist of large, scattered granules 

 (figs. 106-109, 112-114). Mitosis may be halted at any stage for an indefinite 

 period, the spindles and chromosomes remaining distinct, while the centrosomes 

 become larger than normal and the polar rays grow indistinct and finally disap- 

 pear entirely (figs. 107, 109). When mitosis has been halted for a long time, as 

 in the egg shown in fig. 116 which was in a stoppered tube of boiled sea water for 

 48 hours, the spindles shrink in size and stain deeply, while the chromosomes 

 form a distinct ring around the spindle. 



2. Atmosphere of Hydrogen. 

 (Figs. 110, 111, 115, 117, 118. Exps. 879-883, 1023-1029.) 



Hydrogen was generated in the well known Kipp apparatus and a stream 

 of the gas, after being purified and washed, was allowed to bubble through the 

 sea water in a flask containing the eggs to be studied. In this way, most of the 

 air in the water and flask is replaced in a short time by hydrogen. 



The first effect of such treatment is a slowing of division; after 4J^ hours, 

 there were 1-cell and 2-cell stages present in the flask, whereas in the control all 

 the eggs had passed beyond these stages; after 73^ hours, there were many 1-cell 

 to 4-cell stages present in the flask, whereas in the control all had reached the 

 24-cell stage. Development in an atmosphere of hydrogen is normal as far as it- 

 goes; the direction, size and quality of cell division and the direction of move- 

 ment in telokinesis are not affected, but it is quite evident that after a short time 

 the eggs cease altogether to develop in an atmosphere of hydrogen. In another 

 experiment (No. 1023), hydrogen was allowed to bubble for 2 hours through 

 a wash bottle containing eggs and the bottle was then left open to the air for 2 

 hours. Most of these eggs appeared quite normal, though a few showed swollen 

 nuclei; however, development had been stopped. In other experiments (Nos. 

 1024-1026), hydrogen was run through bottles containing eggs for 1 or 2 hours 

 and the bottles were then stoppered and the eggs left in them for 3, 5, 18, or 

 20 hours. When the eggs were in an early stage at the time of the experiment, 

 development was stopped; in one case (No. 1026), where the eggs were in an 

 advanced stage of cleavage when the experiment began, development progressed 

 normally until normal gastrulse were formed 20 hours after. This one experiment 

 indicates, what many others prove, that the early stages of cleavage are much 

 more sensitive to environmental changes than are the later ones. In all cases 

 in which development is stopped for some time, nuclei and nucleoli become 



