MUSEUM OF COMPARATIVE ZOOLOGY. 575 



a strongly colored corpuscle, which is surrounded by a clear area and rays. In 

 approaching the female pronucleus, the corpuscle increases to nearly double 

 its original size. Preparations made when the female pronucleus is already 

 surrounded by rays of the male aster show that the nucleus is almost con- 

 stantly oval, and drawn to a point on the side nearest the male pronucleus. 

 The nature of the male pronucleus is especially elucidated in the star-fish. 

 It is sometimes only as large as that of the sea-urchin, but at other times 

 twice as large ; in the latter case it no longer has a homogeneous appear- 

 ance, but is surrounded by an enveloping layer which is darker than the 

 contents. The cause of this difference is unknown, but it establishes a transi- 

 tion between the condition shown by the sea-urchin and that of the Hetero- 

 poda where the two pronuclei have the same size and texture. Were it not 

 for this transition, it would be difficult to ascertain whether the dark cor- 

 puscle of the sea-urchin corresponds to the pronucleus of the Heteropoda or 

 only to its nucleolus. 



Besides the results already (p. 479) mentioned, the study of the pronu- 

 clei in Sagitta have afforded other points of particular interest. The altered 

 form, which I ventured to assume for the male pronucleus, is actually encoun- 

 tered, and corresponds almost exactly with that of the pronuclei in Limax, 

 Fig. 68. I extract the following from the author's description of the changes. 

 Although fecundated at the moment of deposit, the vitellus shows a male 

 aster only at the time when the polar globules are formed. It probably exists 

 already at the edge of the yolk, but it must be quite small, since it escapes 

 observation. Soon after the elimination of the globules there appears near 

 the surface of the yolk, usually at the nutritive pole, a round or oval vacuole, 

 the male pronucleus. The female pronucleus appears almost at the same in- 

 stant. They iiiove toward the centre, increasing in size, and meet between it 

 and the formative pole. The female pronucleus is without an aster, that of 

 the male grows rapidly, and lies in advance of the pronucleus. The cavity of 

 the male vacuole is surrounded hy a sharp margin, except at the place where it 

 touches the centre of the aster. There it appears open, as though the contents of the 

 cavity passed by gradations into the substance forming the central mass of the aster. 

 The vacuole always assumes the form of a mslon-seed. This description corre- 

 sponds in almost every particular with the condition in Limax alluded to above. 

 The author, believing from the appearance that the pronucleus is drawn on in 

 a passive condition, and that the agency must be sought in the male aster, 

 endeavored to show by reagents the presence of the body of a zoosperm, or a 

 compact corpuscle, in the centre of the aster. Failing in this, "he must con- 

 sider the vacuole and the central mass of the aster taken together as the homo- 

 logue of the male pronucleus of other animals." This conclusion, if extended 

 to the cited case in Limax, would involve one in the necessity of identifying 

 the central area of the same aster with both male and female pronucleus ; and 

 in Sagitta certain stages in the approximation of the pronuclei {op. cit, Taf. X. 

 Fig. 7) appear to present the same difficulty, for the relation of the female 

 pronucleus to the aster is at this stage essentially the same as that of the male 



