MUSEUM OF COMPARATIVE ZOOLOGY. 145 



But why one nucleus should become altered in one way, and the other 

 in an entirely different manner, is difficult to say. A very similar dif- 

 ferentiation of nuclei has been observed by Chun ('90) in the egg germs 

 of a Siphonophore {Stephanophys). He found only one nucleus in the 

 youngest germs, while the middle-sized and larger egg cells contained 

 two of different size, the larger being pale, and the smaller staining in- 

 tensely. The smaller nucleus moves to the periphery of the egg and is 

 no longer visible when the latter is ripe. The larger nucleus persists 

 as the germinative vesicle. In only one instance did he see a stage 

 that showed that the smaller nucleus budded out of the larger. Chun 

 compares the small, deeply staining nucleus to the " StofFwechselkern" 

 (macronucleus), and the pale one to the " Fortpflanzungskern " (micro- 

 nucleus) of the ciliate Infusoria. 



Summary. 



1. The embryo of the scorpion is enveloped by three membranes, the 

 ovarian capsule, the serosa, and the amnion. 



2. The ovarian capsule is an enlargement of the ovarian tube ; the 

 serosa and amnion arise from the blastoderm of tlie egg. 



3. Serosa and amnion are at first distinct, and joined to each other 

 by minute fibres. These afterwards disappear, and the membranes 

 coalesce. 



4. The serosa is composed of immense flat cells, very variable in size 

 and shape. The cell walls are fibrilJated. 



5. The majority of the serosa cells have two large nuclei of equal 

 size. There are rai'cly more than two. 



6. The nuclei are disk-shaped, have a distinct nuclear membrane, and 

 chromatin in the foi-m of granules and filaments, the latter forming an 

 indistinct reticulum. There are usually several nucleoli. 



7. The cytoplasm of the serosa has a distinct reticular structure. 



8. Nuclear division in the serosa is amitotic, and takes place by con- 

 striction, preceded by elongation of the nucleus. It is followed or ac- 

 companied by recession of the daughter nuclei, which remain for some 

 time connected by a fine strand. 



9. Constriction of the nucleus is usually accompanied by a longitudi- 

 nal arrangement of some of the chromatic threads, radiating from the 

 constricted part. The nucleoli are distributed about equally to the 

 daughter nuclei. 



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