MUSEUM OF COMPARATIVE ZOOLOGY. 413 



the substance of the vitellus consists of three layers, — superficial, inter- 

 mediary, and central. The second is coarsely and irregularly granular, 

 and more opaque than the other two ; the central is clearer, but 

 uniformly granular ; the superficial is almost homogeneous, very refrin- 

 gent, and contains only punctiform granulations. At a point of this 

 outer layer a thickening occurs, and in this point appears a small, round 

 body, which is destitute of granulations and resembles a vacuole ; but in 

 osmic acid this so-called vacuole darkens and assumes a gray tinge, 

 while the vitellus is colored brown. This is the pronucleus peripherique. 

 This sinks deeper into the yolk, at the same time becoming larger, and 

 there appear within it numerous very refringent corpuscles of variable 

 size which resemble nucleoli. In the " central mass " of the egg there 

 appear simultaneously two or three small clear irregular masses, which 

 directly unite into a body with bunched (bossele) surface. This occu- 

 pies from the first the centre of the eg^. It is called by Van Beneden 

 pronucleus central. It differs from the peripheral pronucleus in being 

 considerably larger, and in having a less distinct contour. The two 

 approach till they touch each other at the middle of the yolk. The 

 peripheral pronucleus is spherical, and its contour is regular. The 

 central has the form of a calotte or of a flattened crescent with hhint 

 horns* its concavity being moulded upon the peripheral pronucleus, 

 from which it is at first separated by central protoplasm sometimes 

 containing several voluminous and refringent granules. In most of the 

 eggs, however, the pronuclei touch or are separated by only an imper- 

 ceptible layer of vitelline protoplasm. The convex face of the central 

 pronucleus is sometimes regular, sometimes lobed, and occasionally 

 divided into two parts in such a manner that there are three conjoined 

 clear bodies. The substances of central and peripheral pronucleus are 

 optically alike, and both exhibit rounded refringent corpuscles of vari- 

 able size, — the nucleoli. 



The peripheral pronucleus increases rapidly in size, but preserves its 

 spherical form. The central diminishes in volume. They become much 

 less apparent, and at length there exists only one nucleus formed at 

 the expense of the two. Whether they fuse, or one is developed at the 

 expense of the substance of the other, the author is unable to say. This 

 nucleus has an irregular form, indistinct contours, and is composed of a 

 homogeneous substance in which nucleoli are not distinfruishable. From 

 the time the pronuclei approach each other in the centre, the vitellus 



* These eggs were treated with 1% osmic acid, put two or three days iu Miiller's 

 fluid, and then mounted iu glycerine. 



