MATURATION OF THE OVUM. 



6 9 



the zona radiata. It contains an eccentrically-situated germinal 

 vesicle and 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 reti- 



FIG. 23. Two SUCCESSIVE STAGES IN THE GRADUAL METAMORPHOSIS OF THE 



GERMINAL VESICLE AND SPOT OF THE OVUM OF ASTERIAS GLACIALIS IMMEDI- 

 ATELY AFTER IT is LAID (copied from Fol). 



culum vanishes, its membrane 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 out- 

 line 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 to- 

 wards the germinal vesicle 1 . At the 

 end of the nuclear spindle may be seen 

 the broken up fragments of the germi- 

 nal spot. 



At a slightly later stage, in the 

 place of the original germinal vesicle 

 there may be observed 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. 



M 



FIG. 24. OVUM OF ASTERIAS GLA- 

 CIALIS, SHEWING THE CLEAR SPACES 

 IN THE PLACE OF THE GERMINAL 



VESICLE. FRESH PREPARATION (copied 

 from Fol). 



