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 the 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, Tho serosa is composed of immense flat cells, very variable in size 
and-shape. The cell walls are fibrillated. 
5. The majority of the serosa cells have two large nuclei of equal 
size. There are rarely more than two. 
6. The nuclei are disk-shaped, have a distinct nuclear membrane, and 
chromatin in the form 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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