^86 WILSON. [Vol. VIII. 



marks the lower pole of the blastiila, sometimes persists up to 

 a stage as late as the gastrula shown in Hatschek's Figs. 26, 

 27. In all such cases I examined it lay exactly at the cetitral 

 point of the dome — a fact that seems to show that the invagina- 

 tion is primarily symmetrical, as originally described by 

 Kowalevsky. Whether this is true of later stages I have not 

 yet certainly determined, but my observations do not accord 

 with those of Lwoff (20). An extensive series of prepa- 

 rations (most of them fixed with picro-sulphuric acid, or with 

 Flemming's fluid, stained with Mayer's hsema-calcium and 

 mounted in balsam) show in the clearest manner that through- 

 out the whole period of gastrulation, up to a stage later than 

 Lwoff 's Fig. I, both macromeres and micromeres are under- 

 going active division. The macromeres like the micromeres 

 show numerous conspicuous mitoses, and in every part of the 

 entoblast plate. Lwoff's assertion, that during the invagination 

 mitoses in the entoblast cells "so gut wie ganz fehlen," must 

 rest upon very incomplete evidence; for in my preparations 

 there is no perceptible difference between the two layers in 

 this regard— though of course mitoses are absolutely more 

 numerous in the ectoblast since the actual number of cells is 



greater. 



Part II. — Induced Forms of Development. 



The blastomeres of the early segmentation-stages of Amphi- 

 oxus may very easily be more or less completely separated from 

 one another by Driesch's ingenious shaking-method, and in 

 their subsequent development give rise to modified forms that 

 are of the highest interest for the general problems of 

 cleavage. The separation of the blastomeres is much, more 

 readily effected than in Echinus, and the induced forms of 

 development may be obtained in great numbers ; but this is 

 offset by the fact that the eggs cannot be fertilized artificially, 

 and the available time for work is, therefore, very limited. My 

 stay in Faro could not be extended beyond six weeks, and it 

 seemed best to devote most of it to the study of the earlier 

 stages and to the preservation of material. For these reasons 

 my work shows large and obvious gaps, and I have been unable 



