KARYOKINESIS. 21 



tion of the spindle, which always ultimately turns so that one pole lies immediately 

 under the first polar body. If one may judge by the figures of many authors, this 

 must be a phenomenon which occurs among a large number of animals. 



4. Polar Bodies. — Finally, the outer half of the mitotic figure, with a small 

 amount of surrounding cytoplasm, protrudes from the general surface of the egg. 

 The furrow separating the first polar body begins to form at the periphery and pro- 

 ceeds toward the middle of the stalk connecting the polar body with the egg. In 

 some cases the spindle seems to retain its full diameter, even when the cytoplasm 

 has been completely constricted by the dividing furrow, fig. 22, as has also been 

 observed by Kostanecki and Wierzejsky ('96) in Physa. Afterward, the spindle 

 itself becomes constricted in the middle, fig. 23 ; and the constricting ring of darkly 

 staining substance finally cuts the spindle in two and itself becomes a spherical 

 mid-body. During and after the separation of the first polar body, one first becomes 

 aware of the fact that there is an egg membrane, which takes no part in the con- 

 striction, but is lifted from the egg by the polar body, Plate II, figs. 22, 23, 28, 30 

 and 31. 



The second polar body is smaller than the first and is separated from the egg in 

 the same way as the first, a mid-body being formed, as shown in figs. 34 to 41. This 

 mid-body is larger and persists longer than that of the first maturation, as Mark and 

 Kostanecki and Wierzejski have also observed. When fully developed, it consists of 

 a central granule and two surrounding spheres, the inner one small, dense and sharp 

 in outline, the outer one large, less dense and indistinct in outline. The remains of 

 the spindle can be seen running through this outer sphere as two cones, their apices 

 being in contact with the inner sphere and their bases with the two nuclei. 



The first polar body divides by mitosis into two, figs. 27, 28#, 32, 34 and 36, 

 and each of these may subdivide amitotically into a large number of cells, some of 

 which are unequal in size and recall the macromeres and micromeres of developing 

 ova, figs. 41, 45, 61, 69, 73 and 81. I have never seen the second polar body 

 dividing. 



II. Fertilization. 



1. Entrance of Spermatozoon. — Copulation occurs only at long intervals, 

 perhaps once in the life time of a female, and the spermatozoa are stored after 

 copulation in a tubular outgrowth of the uterus. Ova and spermatozoa meet in 

 the uterus, and here the entrance of the spermatozoon occurs, though the later 

 stages in the approach of the egg and sperm nuclei do not occur until after the 

 capsules have been formed and deposited. In the examination of thousands of 

 eggs taken from the egg capsules I have never found one which was unfertilized 

 and very few into which more than one spermatozoon had entered. 



A mature spermatozoon is shown in fig. 17; there is a relatively large head 

 with pointed apex, separated, in preserved material, by a clear space from the tail. 

 I am inclined to regard a minute, darkly staining cap which covers the posterior end 

 of the head as the middle piece. It is extremely small and appears to contain no 



