578 BULLETIN OF THE 



pneiistes. After fecundation the vitelliis remains in repose for about twenty- 

 minutes. There is a collection of transparent substance which forms an irregu- 

 lar layer around the central nucleus. The radiations in the yolk appear before 

 the nucleus has suffered reduction of volume ; they are optically like the sub- 

 stance which surrounds the nucleus, not like that of the nucleus itself. They 

 diminish in breadth at the moment the latter is converted into an amphiaster. 

 Auerbach's theory is refuted by these facts. Subsequently the nucleus is a 

 little elongated, and the perinuclear protoplasm takes the form of a disk sur- 

 rounding the nucleus, as the ring of Saturn does its planet. The disk is oval ; 

 when seen in profile the vitelline rays appear to diverge from it like the barbs 

 of a feather. Treated with acetic or picric acid, the radial structure, contrary 

 to the effect produced in subsequent stages, becomes less distinct. This phase 

 lasts about twenty minutes. The protoplasmic disk meantime gradually dimin- 

 ishes in breadth and increases in length. Then it promptly becomes limited to 

 two masses quite distinct from each other. The rays are no longer arranged 

 like barbs, but like the spokes of a wheel. The nucleus becomes indistinct; 

 reagents cause it to reappear in the form of a lemon. At the pointed extrem- 

 ities the limiting layer of the nucleus projects outward, and serves as centres for 

 the two systems of rays. In acetic acid the latter are seen with great distinct- 

 ness ; they are without enlargements, and are soon lost in the midst of vitelline 

 substance of uniform appearance. One or two of the granules in the nucleus 

 are distinguished by their size and greater refringency, — perhaps a nucleolus in 

 process of dissolution. Osmic acid confirms the existence of these conditions. 



Transitions from this to the next described phase are not often met with in 

 the sea-urchin, but are more readily found in the heteropods, in connection 

 with which they are described. 



In this next phase the spindle with equatorial fibre thickenings is already 

 formed. It is during this stage that the second vitelline membrane begins to 

 be detached. After treatment with picric acid there is no trace of an envelop- 

 ing layer (membrane) around the nucleus ; each aster is composed of distinct 

 parts : a central, nearly spherical, clear, protoplasmic mass ; a peripheral gran- 

 ular part, dark especially in the vicinity of the central mass, and of a radial 

 texture remarkable for its delicacy and regularity. The dark substance termi- 

 nates abruptly with a regular contour, but is not separated from the central 

 mass by any membrane or envelope whatever. The limits are less pronounced, 

 but not wanting, on the side toward the old nucleus. The centre of the clear 

 portion is occupied by a cluster of granules, toward which all the filaments are 

 directed. They stop at the edge of the clear mass ; it is exceptional to see a few 

 of the intranuclear filaments send pale prolongations as far as these granules. 

 Rotation of the egg shows that the spindle is flattened so that its cross-section 

 is elliptical, and that the cluster of granules at the centre of the aster has the 

 form of a crescent, and therefore appears in section as a round body of limited 

 extent. Treated with osmic acid, the extranuclear rays are almost obliterated, 

 so that the central mass appears with greater distinctness. In acetic acid the 

 unipolar rays are seen with surprising clearness, and a remnant of the nuclear 



