274 W. G. VanNcime — Embryology of Eustylodcus. 



spindle, suggest that it is formed by a progressive differentiation of 

 the cytoplasm about the centrosome. 



As I have already said, there is at first no central spindle connect- 

 ing the two asters which are to become the poles of the first polar 

 spindle. They seem to be entirely independent (Figs. 2, 3a and 'Ab). 

 But as the nuclear membrane, which has everywhere become very 

 thin and delicate, is more and more completely dissolved, the rays 

 extend into the germinal vesicle and connect with the chromosomes 

 and with rays of the other aster (Fig. 4 and 5). Thus a spindle is 

 formed, the fibers of which are in the early stages very irregular 

 and anastomose with each other in so complex a manner that they 

 appear somewhat like a more condensed and deeply staining part of 

 the general reticulum of the egg. 



As Coe (1) has found in the egg of Cerehratulus, the spindle is thus 

 partly at least formed from material previously or to some extent 

 still enclosed in the nuclear membrane, and therefore undoubtedly 

 nuclear. This is Francotte's opinion also. Van der Stricht expresses 

 the belief that a rudiment of the central spindle exists from the 

 time the centrosomes first separate,^ but admits that no such thing is 

 visible in the early stages. It is probable, however, that if such a 

 structure did exist and were sufficiently substantial to persist while 

 the centrosomes move to opposite sides of the germinal vesicle, it 

 would at the same time be distinct enough to be visible. 



The development thus far takes place usually in the ovaries or at 

 least in the first portions of the oviducts. Francotte however men- 

 tions cases where the egg was laid while still in the germinal vesicle 

 stage. Such instances I have not met with. By the time the egg 

 reaches the uterus (Fig. 5) the first polar spindle is fully formed, 

 the chromosomes lying in or near its equator, though some of them 

 are often more or less displaced (Fig. 36). Their form is now 

 usually that of a ring, a cleft or opening having appeared through 

 the center of each. This ring form is sometimes visible at a still 

 earlier stage (Fig. 3a). The other forms which the chromosomes 

 may assume are described below. 



About the time the nuclear membrane dissolves the nucleolus dis- 

 appears also. Occasionally there are traces of it for a little while 

 afterwards both in Eustylochus and Planocera. Francotte observed 

 this in Leptoplana and Prosthecerceus, but Van der Stricht found 

 that it did not persist after the disappearance of the membrane in 

 Thysanozooji. 



By this time the aster rays have increased both in length and in 



