ovarV and ovarian ova in certain marine fishes. 139 



visible, and stains scarcely at all. It is this condition which 

 has led to the conclusion that the chromatic network dis- 

 appears ; but Riickert says that the chromosomes are converted 

 into much expanded structures, consisting of numerous trans- 

 verse threads along an axis, so that each chromosome resembles 

 the cylindrical brush used for cleaning lamp chimneys. The 

 change is effected by the microsomes first becoming elongated 

 into transverse rods, or perhaps discs, and then further breaking 

 up into the thin curved threads. In eggs | to 1 mm. in diameter 

 the tangle of chromosomes again becomes more distinct. To 

 see the general distribution of the chromosomes, whole germinal 

 vesicles must be isolated and examined, because a section 

 takes in only a very thin slice of the large structure. It is 

 now found that the chromosomes are arranged in pairs, and 

 there are as many pairs as there were chromosomes in the 

 earlier stage, which shows that each pair has been produced 

 by the division of a chromosome. The position of the members 

 of each pair indicates that the division of the chromosome has 

 been longitudinal. 



The second period of the development is to be followed in 

 eggs from 2 or 3 mm. in diameter to the full-grown condition 

 when they are 14 to 16 ram. in diameter. The vesicle becomes 

 smaller, and reaches the surface of the egg, against which 

 it flattens itself to some degree. The chromosomes remain 

 in pairs, but become much shorter and thinner, and they 

 concentrate towards the centre of the vesicle. 



In eggs of 14 mm. diameter the coil or tangle formed by the 

 chromosomes is only 36 ^ by 8 /^ in dimensions, while the whole 

 vesicle is 296 fx by 148 fx. The external part of the feathery 

 structure of the chromosomes becomes stainless, while the 

 central part stains more deeply. Riickert regards the changes 

 in this and the preceding period as to a great extent due to a 

 divergence and convergence of particles. Finally, the chromo- 

 somes become converted into portions of chromatin, consisting 

 of distinct granules, but having the form of dense masses, not 

 elongated loops or cords; in some of the masses there are 

 indications that each consists of four rods. 



