W. G. Van Name— Embryology of Eustylochus. 273 



sometimes seen more or less suggestion of such a " couche cortical," 

 but cannot consider it as anything but an abnormal or artificial 

 effect, as the best preparations do not show it. An appearance 

 which may be of somewhat the same nature is very frequent in eggs 

 where the penetration of the killing fluid has been slow or imper- 

 fect. No centrosome can be found in such eggs and the centro- 

 sphere is indefinite in outline and surrounded by a light colored 

 layer of greater or less thickness, in which no structure can be 

 discerned. The aster rays begin outside of this. Such a condition, 

 although unquestionably abnormal and artificial, I have represented 

 in Figs. 24 and 28, as other structures in the Qgg are well shown in 

 these specimens. It occurs both in the polar and cleavage spindles. 



Francotte (4) illustrates in his Figs. 2*7, 29, 30 and 31, sections 

 which apparently have this same defect, and it is lai-gely upon such 

 that he appears to base his belief that the centrosphere (he calls this 

 with the " corpuscle central " the " centrosome") is surrounded by a 

 membrane in the later phases of mitosis. Such a condition is not 

 indicated by my best preparations, though it is probably true that 

 the outer portion of the centrosphere becomes more dense. At least 

 it stains more deeply. (See Figs. 5, 7, 8, 12 and 13.) 



That which I have termed centrosome is the "granule central" 

 or " corpuscule central " of Van der Stricht and Francotte, the 

 " cenfralkorn " of various writers. 



Van der Stricht (28) believes that this, as well as the centrosphere 

 ("couche medullaire "), is formed by differentiation out of a "cen- 

 trosome" which is at first homogeneous. Moreover he considers 

 that although the "sphere attractive," which includes what I have 

 termed the centrosphere, may not always reveal itself with all the 

 clearness that might be desired, yet it is none the less a permanent 

 organ of the cell. 



This view is certainly not supported by the subsequent history of 

 the centrosphere in Eustylochus or Planocera. As explained below, 

 the greater part of it is destined to form the second maturation 

 spindle, and the centrosphere that remains in the egg after the sec- 

 ond polar body is expelled rapidly degenerates and disappears. The 

 manner in which the centrosphere is formed is a much more difiicult 

 matter to determine, but the poorly defined limits of the centro- 

 sphere in the early stages both of the polar and cleavage spindles, 

 where it runs out into the rays with no distinct line of demarcation 

 between the latter and the centrosphere, when compared with its 

 distinct and definitely limited outline in the later stages of the same 



Trans. Conn. Acad., Vol. X. August, 1899. 



IS 



