No. I.] THE ECHINODERM EGG. 333 



indicating that the action of the sperm upon the ^^^ depends 

 upon the introduction of a substance into the latter. In one 

 case only twelve per cent of eggs matured when unshaken, 

 whereas over fifty per cent were induced to mature by a little 

 agitation; and, had the shaking been more violent or more 

 prolonged, similar experiments upon other eggs prove that the 

 proportion would have been increased. Sections of these eggs 

 show that the most noticeable result of the operation is the 

 rupture of the membrane of the germinal vesicle, and this 

 appears to be essential to maturation. This tearing of the 

 membrane permits the escape of nuclear matter into the cyto- 

 plasm, and there is evidence that a substance is at this time 

 set free which, either directly or indirectly, forms the centro- 

 somes. In ovarian normal maturation the wall of the germinal 

 vesicle is dissolved, and finally breaks at a point nearest the 

 surface of the Q%g. At this point the centrosomes are invari- 

 ably developed. In eggs matured artificially without the aid 

 of shaking the membrane becomes first wrinkled where it lies 

 nearest the surface of the &^g^ and here, also, the centrosomes 

 are formed. 



I. Origin of the Centrosomes. 



The fate of the centrosome derived from the egg mother-cell 

 has not been accurately determined on account of the minute- 

 ness of the elements. During the skein stage of the chromatin 

 of the very young egg, however, a small cytoplasmic accumu- 

 lation like a cap is generally present at one end of the nucleus. 

 This resembles the archoplasm, or " Nebenkern," as often 

 figured. Whether it be the remnant of the spindle or not, it 

 bears no constant relation to the place of formation of the 

 nucleolus, and quickly disappears. No centrosome is thereafter 

 to be found until the time of maturation. 



At maturation the centrosomes are first accurately to be 

 distinguished as two (at a very early stage apparently one) 

 deeply staining, small, but distinct and characteristic, granules 

 lying side by side either in the nuclear membrane or immedi- 

 ately without it, and invariably on that part of the vesicle 

 nearest to the surface of the Qgg. Occasionally one of these 



