MUSEUM OF COMPARATIVE ZOOLOGY. 129 



and a transverse section (see Fig. 2) shows only a trace of a dividing 

 wall between them, although in surface view the cell walls of both mem- 

 branes are clearly seen (Fig. 1). Metschnikoff (71, p. 219) describes 

 the membranes of Scorpio (Euscorpius) italicus as connected with each 

 other by delicate fibres, which terminate just over the amniotic nuclei. 

 I have found such fibres in the earlier stages of my material, but not in 

 the older ones, nor are they everywhere present in the younger mem- 

 branes. The membranes of the Brazilian scorpion examined by Bloch- 

 mann ('85, p. 481) were found closely applied to each other. 



I. The Serosa. 



Plate I. ; Plate II. Figs. 14, 15 ; Plate III. 



The cells of the serosa have great superficial extent, measuring 

 half a millimeter or more in diameter ; but proportionally they are 

 very thin. Their size is exceedingly variable, as may be seen by com- 

 paring Figure 3 with Figures 11 and 13 of the same magnification, 

 although the last two represent cells of only average size. Both 

 small and large cells are apt to be aggregated in certain parts of the 

 serosa, yet very small cells often occur sporadically in the midst of 

 large ones. The cell walls are extremely distinct in late stages of the 

 embryo, but in earlier stages are often difficult to trace in an ordinary 

 stained preparation. As remarked by Blochmann, they have a distinct 

 fibrous structure. The cells are irregularly polygonal in shape, usually 

 elongated, sometimes nearly square or triangular. Not infrequently 

 they are bounded by curved outlines (Fig. 13). 



The nuclei of the serosa measure from 25 ^ to 60 /* or more in 

 diameter, but as a rule are small in proportion to the cells (Figs. 1-3 

 and 11-15). In the membranes of young embryos the nuclei are larger 

 absolutely and in proportion to the cells than in old membranes. In 

 face view the resting nucleus is nearly circular ; in section, it is seen 

 to be considerably flattened, in accordance with the thinness of the cell 

 (Fig. 2, nl. sr.). It occupies the full thickness of the serosa, and some- 

 times causes a bulging of the cell at the point where it lies, as is shown 

 in Figure 2. Blochmann states ('85, p. 480) that the nuclei of the se- 

 rosa always cause that membrane to encroach inward upon the amnion ; 

 but a dividing line between amnion and serosa is so seldom visible in 

 Centrurus, that I am unable to say whether such is the case. 



The nuclear membrane is thin, but clearly visible, except in nuclei that 

 have undergone degeneration. The chromatic substance, or nuclein, is 



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