ARTIFICIAL PARTIIENOGKNLISIS AND FEUTILTSATION. 489 



no cytoplasmic cleavage takes place. The cleavage ceutro- 

 soiiies first make their appearance outside the nucleus, but 

 directly on the nuclear membrane, and the evidence renders 

 it nearly certain that they arise by the division of a single 

 primary egg centrosorae that is formed de novo. All the 

 evidence goes to show that the cleavage centrosomes are of 

 the same general nature as the central bodies of the cytasters." 



Among many interesting details I will refer here only to 

 the changes described for eggs which underwent segmenta- 

 tion^ and were capable of developing into swimming embryos, 

 because iu certain particulars they are reminiscent of what 

 takes place in the formation of the first polar amphiaster. 



I may summarise as follows : — (1) The first change that 

 occurs is a coarsening in the appearance of the protoplasm, 

 better marked in eggs treated by stronger solutions. (2) A 

 primary radiation appears centering on the nucleus, better 

 marked in eggs treated with weaker solutions. (3) A varying 

 number of secondary radiations appear in eggs especially 

 treated with stronger solutions. The extent of the primary 

 radiations is inversely in proportion to the number of the 

 secondary radiations. These latter appear as vague clear 

 spots iu the cytoplasm, which gradually become surrounded 

 with radiations, and finally assume the form of asters. They 

 always appear in situ, and do not change their position till a 

 later period. (4) Coincident with the appearance of the 

 radiations there is a gradual growth of the nucleus. (5) 

 Round the nucleus appears a clear perinuclear zone of hyalo- 

 plasm. (G) The nuclear 7nembrane fades out, and a vague 

 irregular clear space is left, to which the hyaline zone con- 

 tributes. (7) The rays then diminish, and, indeed, almost 

 disappear. 



The eggs at this point were restored to pure sea water, and 

 after a pause the radiations reappear and advance centri- 

 fugally towards the periphery. In eggs capable of develop- 

 ment the principal rays are now focussed on two centres at 

 opposite poles of the nuclear area, which now forms a spindle 

 connecting the two asters. If the amphiaster is typical. 



