MUSEUM OF COMPARATIVE ZOOLOGY. 313 



remain in contact by that part of their surfaces which is developed by 

 the division of the cell-plate. The "pronucleus derive" increases by 

 blending with the "pronucleus engendre." The part of the old nucleus 

 which remains striate and non-granular continues adherent to the cell- 

 plate. The young nucleus enlarges, becomes clearer and more central 

 in position, its contour regular, and it acquires a small nucleolus. Finally 

 the last trace of the striated part of the old nucleus disappears. 



Rabl ('76, p. 318) often observed a karyolitic figure, especially in eggs 

 (Unio) hardened by chromic acid, but is unable to give any account of 

 the nuclear division itself. 



In Bobretzky's " Studies on the Embryonic Development of Gastero- 

 poda " ('76), some attention is given to the changes of the nucleus dur- 

 ing segmentation, especially in the case of Nassa mutabilis. Lam. (pp. 97 - 

 102). Usually the nucleus is not to be found after the egg is laid, but 

 in one instance, by employing pressure, it became visible immediately 

 under the surface of an egg which had already given rise to polar glob- 

 ules. Its contents had a homogeneous, water-clear appearance. Subse- 

 quently, there is a lengthening of the egg in the direction of the diameter 

 passing through the formative pole, and, following this, a constriction in 

 a plane at right angles to this diameter. At this time no nucleus is 

 discoverable in either half of the incompletely segmented egg. Thin 

 sections through the upper (or formative) half of specimens hardened in 

 chromic acid exhibit the radial figures seen by Fol and others. In the 

 middle of the finely granular substance are to be seen two clear spots 

 without granules, from which there diverge toward the periphery rays 

 which are formed of granules abutting upon each other to form straight 

 lines. Those of the rays which are directed from the two centres toward 

 each other unite midway between the spots, and such as lie on either 

 side of a line uniting the latter are more or less curved. In the middle 

 this system of curved lines is interrupted by a very narrow streak of 

 highly lustrous granules, which are somewhat coarser than those compos- 

 ing the balance of the lines. This is Biitschli's spindle and its equatorial 

 zone. Although on sections, the spindle stands forth somewhat more 

 sharply than the radial rays, and is a little more intensely stained in 

 indigo-carmine, yet it is of quite the same nature. Not only is there 

 no nuclear membrane, as Biitschli thinks, but, says Bobretzky, "I cannot 

 distinguish in it [spindle] the actual fibres, which, just like the rays, are 

 nothing else than serially arranged granules." The appearance of two 

 radial centres precedes the origin of the spindle-shaped body. In later 

 stages of segmentation the two stars were already to be seen when the 



