MATURATION OF THE OVUM. 



57 



FIG. 25. OVUM OF ASTERIAS GLACIALIS, 

 AT THE SAME STAGE AS FIG. 24, TREATED 

 WITH PICRIC ACID (copied from Fol). 



ovum now contains a round body, fig. 25. Fol concludes that the spindle is 

 formed out of part of the germinal 

 vesicle and not from the germinal spot, 

 while he sees in the round body pre- 

 sent in the lower of the two clear 

 spaces the metamorphosed germinal 

 spot. He will not, however, assert 

 that no fragment of the germinal spot 

 enters into the formation of the spin- 

 dle. 



The following is Hertwig's (No. 

 92) account of the changes in the 

 germinal vesicle in Asteracanthion. 

 Shortly after the egg is laid the proto- 

 plasm on the side of the germinal vesicle 

 towards the surface of the egg develops 

 a prominence which presses inwards the wall of the vesicle. At the same 

 time the germinal spot develops a large vacuole, in the interior of which is a 

 body consisting of nuclear substance, and formed of a firmer and more refrac- 

 tive material than the remainder of the germinal spot. In the prominence 

 first mentioned as projecting inwards towards the germinal vesicle first 

 one star, formed by radial striae of protoplasm, and then a second make their 

 appearance; while the germinal spot appears to have vanished, the outline 

 of the germinal vesicle to have become indistinct, and its contents to have 

 mingled with the surrounding protoplasm. Treatment with reagents demon- 

 strates that in the process of disappearance of the germinal spot the nuclear 

 mass in its vacuole forms a rod-like body, the free end of which is situated 

 between the two stars which occupy the prominence indenting the germinal 

 vesicle. At a later period granules may be seen at the end of the rod and 

 finally the rod itself vanishes. After these changes by the aid of reagents 

 there may be demonstrated a spindle between the two stars, which 

 Hertwig believes to grow in size as the last remnants of the germinal spot 

 gradually vanish, and he maintains that the spindle is formed at the ex- 

 pense of the germinal spot. The stage with this spindle corresponds with 

 fig. 25. 



Several of Hertwig's figures closely correspond with those of Pol, and 

 considering how conflicting is the evidence before us, it seems necessary 

 to leave open for Asterias the question as to what parts of the germinal 

 vesicle are concerned in forming the first spindle. 



A clearer view of the phenomena which take place at this stage has 

 been obtained by Fol in the case of Heteropods (Pterotrachsea). In 

 the ovum a few minutes after it has been laid the germinal vesicle 

 becomes very pale, and two stars make their appearance round a 

 clear substance near its poles. The nucleus itself is somewhat 

 elongated, and commences to exhibit at its poles longitudinal strice, 

 which gradually extend towards the centre at the expense of the 

 nuclear reticulum, from a metamorphosis of which they are directly 

 derived. When the strise of the two sides have nearly met, thicken- 

 ings may be observed in the reticulum between them, which give rise, 

 where the striae of the two sides unite, to the central thickenings 



