MUSEUM OF COMPARATIVE ZOOLOGY. 231 



confluence of the thickenings in which new nucleolar structures have 

 arisen, I am unable to determine. The fact that no cases of entirely 

 homogeneous imclei have come under observation, does not seem favor- 

 able to the latter hypothesis, and yet the same method doubtless 

 prevails here as in the formation of the female pronucleus. Treat- 

 ment 'with acetic acid has invariably resulted in a shrivelled appear- 

 ance of the nuclei. With the deepening of the constriction from the 

 animal pole, the two nuclei assume a lengthened form, and take posi- 

 tions such that their long axes correspond approximately with the 

 trend of the interzonal filaments. Toward the close of the constriction 

 (Fig. 91) each of the nuclei has attained a length of a quarter to a 

 third the diameter (ca. 35 /x) of the resulting cleavage spheres. Its 

 breadth is not more than half its length. It is somewhat more convex 

 on the face which looks toward the animal pole. Double contour lines 

 are more manifest with increasing size. Both the number and mag- 

 nitude of the nucleolar bodies increase. They now number twenty 

 or thirty. They vary in size from 3 /x or 4 /x to less than 1 /x, and are 

 nearly spherical. 



When the constriction has advanced from the animal pole to the 

 centre of the yolk, the interzonal filaments are most conspicuous exactly 

 opposite the constriction, where they appear somewhat thickened. These 

 thickenings, however, are never abrupt, but taper gradually on both 

 sides, till, in the vicinity of the nuclei, the filaments become quite faint. 

 As the constriction advances further, this indistinctness affects more and 

 more the terminal portions, till at length only very short, rapidly tapering 

 threads are seen. These, however, persist for some time ; they may 

 even be discovered near the surface of the yolk after tjie two spheres 

 have become detached from each other, and have entered upon other 

 phases of relationship. These thickenings are doubtless equivalent to 

 the so-called " cell-plate " of Strasburger (Zellplatte). The last point of 

 union between the halves of the first segmentation sphere is marked by 

 this remnant of the interzonal filaments ; it is they which are last to 

 yield to the force that divides this first cell into two. 



There remains still another point to be mentioned in connection with 

 the phenomena presented by eggs hardened during the process of cleav- 

 age. It often happens that this treatment causes a thin, superficial layer 

 of the yolk to become separated from the rest of the vitellus on that 

 side where the constriction is farthest advanced (Figs. 90, 93). This ap- 

 pears as a homogeneous structure, less than 1 ^^ in thickness. The 

 inner surface is less sharply marked than the outer, and from the fact 



