No. 2.] THE OVARIAN EGG OF LIMULUS. 157 



become imbedded in a more or less viscid (or granular), stain- 

 able substance. In this substance the nucleolus makes its 

 appearance, usually close to the periphery of the nucleus (PI. 

 XIV, Fig. 39). At first it is often flattened on the side ne.xt 

 to the wall of the nucleus, but elsewhere spherical, though at 

 times irregular in outline. In this early stage it often appears 

 to consist of granules. How much of this granular substance 

 may be due to the reagents cannot be definitely ascertained. It 

 soon becomes homogeneous and spherical, and takes up little 

 by little a more central position (PL XIV). At a very early 

 period in its history it can be seen to be differentiated into an 

 outer and an inner zone (PI. XIV, Figs. 34, 44). The outer 

 zone seems more dense, and at first it seems like a compara- 

 tively thick investing coat of the internal central body. Occa- 

 sionally two nucleoli of essentially similar appearance can be 

 seen in this stage (PI. XIV, Fig. 26). In both the investing, 

 homogeneous layer can be seen to be thinned off at one point, 

 so that the internal, central spherical body partly protrudes 

 through the homogeneous covering. 



One of these nucleoli usually increases more rapidly in size, 

 and later becomes the only one visible. The growing nucleolus 

 becomes the future " Hauptnucleolus," of which there is usually 

 only one, but in some cases two. 



As the nucleolus grows, it retains for a considerable time its 

 spherical form ; and throughout the first period usually remains 

 more or less homogeneous, with now and then spherical vac- 

 uoles in its substance (PI. XIV, Fig. 44; PI. XV, Figs. 83, %7 ; 

 PI. XVI, Figs. 99, 100). These vacuoles are not always mere 

 cavities or fluid particles ; they may contain solid bodies that 

 stain a deep black in Heidenhain's iron-haematoxylin. 



In the second stage of the egg the nucleolus, although it 

 often seems homogeneous, and filled with vacuoles of different 

 sizes, is seen to possess, in a great many cases, a dense outer 

 layer enclosing a central, spherical mass (PI. XV, Fig. 68 ; PL 

 XIII, Figs. 8, 9; PL XVI, Figs. 108, 115, 117, 118. The 

 central mass often has an excentric position, so that the outer 

 homogeneous part, in optical section, has the form of a cres- 

 cent. This can be distinctly seen in the living egg (PI. XVI, 



