GENERAL LVTRODUCTION. 



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a phenomenon \vhi:h occurs before or very soon after the entrance of the spermatozoon. The polar bodies are two minute 

 cells budded forth from the egg, one after another (Text-fig. VII.), which soon perish and disappear without entering into the 

 formation of the embryo. Their precise meaning is not yet fully understood, but their formation is undoubtedly, in some 

 manner, a preparation of the egg-nucleus for union with the sperm-nucleus, since it involves a reduction in the number of 

 chromosomes to one-half the number characteristic of the ordinary tissue-cells of the sjaecies. 



In Toxopneustes and other echinoderms the polar bodies are formed before fertilization, while the eggs are still 

 contained within the ovary; and the same is true of the mammalian ovum, and many others. In many animals, however 

 (e.g., in mollusks and annelids), the polar bodies are not formed until after the spermatozoon has entered the egg. In such 

 cases the sperm-nucleus remains nearly quiescent near the centre of the egg, while the polar bodies are forming 

 (Text-fig. III.). 



The formation of the polar bodies is a true process of cell-division, a typical karyokinetic figure being formed which 

 takes up a radial position near the egg-periphery, and 

 results in the division of the egg into two extremely 

 unequal parts. After the form tion of the polar bodies 

 the egg-nucleus is reformed as a spherical vesicle, much 

 smaller than the original nucleus (germinal vesicle), and is 

 then ready for union with the sperm-nucleus. 



In all cases that have been carefully investigated the 

 number of chromosomes entering into the formation of 

 the polar karyokinetic figure I's, one-half the iisual luimber 

 characteristic of the tissue-cells of the same species. In 

 most cases, but not in all, each of the chromosomes of the 

 first spindle is, from the first, divided into four parts, which 

 form a cjuadruple group, or tetrad ''" Vierergruppe," of 

 German authors). In the formation of the first polar body 

 each of the tetrads is halved to form two double-groups, 

 or dyads (" Zweiergruppen "), one of which remains in the 

 egg while the other goes into the polar body. In the 

 formation of the second polar body each dyad is halved 

 to form two single chromosomes, one of which remains 

 in the egg while its sister passes into the polar body 

 (Text-fig. VIII.). It follows, accordingly, that both the 



egg and the second polar body receive each one-half the number of single chromosomes characteristic of the 

 ordinary somatic cells, while the first polar body receives the same number of dyads or double chromosomes. In some 

 cases the first polar body afterwards divides into two; and it is probable that when this takes place each of the dyads is 

 halved, the two daughter-cells receiving each the same number of single chromosomes as the &^'g or the second polar 

 body. These facts render it highly probable that the formation of the polar bodies is an expression of the means by 

 which the normal number of chromosomes is reduced to one-half in the egg. A very jDrecise parallel to these phenomena 

 occurs in the formation of the spermatozoon. Here, in like manner, the reduction of the chromosomes to one-half the 

 usual number is effected in the course of the last two divisions before the spermatozoon is finally matured. The mother- 

 cell (spermatogonium) gives rise to quadruple chromosomes of one-half the usual number, which are twice halved in the 



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Fig. VIII. 



Fig. VIIT. — Diagrams to show some of the essential facts in the formation of the polar 

 bodies. The normal number of chromosomes is supposed to be six. 



A. Germinal vesicle containing three tetrad-groups (half the normal number of chro- 



mosomes). These have been derived from a portion of the chromatin of the 

 germinal vesicle; the remaining portion degenerates and is absorbed. 



B. The first polar amphiaster with the tetrad-groups (cf. Phototype 3). 



C. Separation of the first polar bodies containing three dyad-groups (each one-half of a 



tetrad-group). 



D. The first polar body formed. Second polar amphiaster. 



E. Separation of the second polar body with three single chromosomes, leaving three in 



the egg. 



F. Both polar bodies formed. The egg-nucleus (E) reconstituted out of the three 



remaining chromosomes (each of them one-fourth of an original tetrad-group). In 

 fertilization the spermatozoon will bring in three chromosomes, restoring the total 

 normal number of si.x. 



