56 



GERMINAL VESICLE. 



a germinal spot. In the former is present the usual protoplasmic 

 reticulum. As soon as the ovum reaches the sea-water the germinal 

 vesicle commences to undergo a peculiar metamorphosis. It exhibits 

 frequent changes of form, the reticulum vanishes, its membrane 



FIG. 23. Two SUCCESSIVE STAGES IN THE GKADUAL METAMORPHOSIS OF THE GERMINAL 



VESICLE AND SPOT OF THE OVUM OF AsTERIAS GLACIALIS IMMEDIATELY AFTER IT IS 



LAID (copied from Fol). 



becomes gradually absorbed, its outline indented and indistinct, 

 and finally its contents become to a certain extent confounded with 

 the vitellus (fig. 23). 



The germinal spot at the same time loses its clearness of outline 

 and gradually disappears from view. 



At this stage, and between it and the stage represented in fig. 26, 

 the action of reagents brings to light certain appearances the nature 

 of which is not yet fully cleared up for Asterias, which have been 

 described somewhat differently by Fol for Ast. glacialis and Hertwig 

 for Asteracanthion. 



Fol finds immediately after the stage just described that a star is visible 

 between the remains of the germinal vesicle and the surface of the egg, 

 which is connected with an imperfectly- formed nuclear spindle extending 

 towards the germinal vesicle 1 . At the end of the nuclear spindle may be 

 seen the broken up fragments of the germinal spot. 



At a slightly later stage, in the place of the original germinal vesicle 

 there may be observed in the fresh ovum two clear spaces (fig. 24), one 



ovoid and nearer the surface, and the 

 second more irregular in form and situ- 

 ated rather deeper in the vitellus. In 

 the upper space parallel striae may be 

 observed. By treatment with reagents 

 the first clear space is found to be 

 formed of a horizontally-placed spindle 

 with two terminal stars, near which 

 irregular remains of the germinal spot 

 may be seen. Slightly later (fig. 25) 

 there may be seen on the lower side of the 

 spindle a somewhat irregular body, which may possibly be part of the remains 

 of the germinal spot, though Fol holds that it is probably part of the mem- 

 brane of the germinal vesicle. The lower clear space visible in the fresh 



1 By the term 'nuclear spindle' I refer to the peculiar form of a double striated cone 

 assumed by the nucleus just before division, which is no doubt familiar to all my 

 readers. I use the term star for the peculiar stellate figure usually visible at the poles 

 of the nuclear spindle. For a further description of these parts the reader is referred 

 to Chapter iv. 



FIG. 2-i. OVUM OF ASTERIAS GLACIA- 

 LIS, SHEWING THE CLEAR SPACES IN THE 

 PLACE OF THE GERMINAL VESICLE. 



FRESH PREPARATION (copied from Fol). 



