444 



SCIENCE 



[N. S. Vol. XXVII. No. 690 



have been expected— (1) in the first ma- 

 turation spindle tetrads, of which there are 

 only twenty present; and (2) in the second 

 maturation spindle dyads. Each dyad is, 

 of course, composed of two monads, but the 

 monads composing a dyad are often 

 deeply constricted, each having a marked 

 dumb-bell shape, so that the whole dyad 

 simulates a tetrad. Since all dyads do not 

 necessarily show this dumb-bell condition 

 of their component monads, the counting 

 is rendered difficult; but the recognition 

 of the possibility of this condition is essen- 

 tial to a satisfactory counting of the 

 chromosomes. These the author maintains 

 are twenty, not twenty-four. Prepara- 

 tions showing the conditions described and 

 illustrated on the blackboard were shown. 

 The author also is of opinion that the 

 first polar cell gradually atrophies, and by 

 gradual diminution in size finally disap- 

 pears. 



The Maturation of the Egg of the Rat: 



"W. R. CoE, Tale University. 



The eggs both of the common brown 

 rat and its albinie variety have been 

 studied in numerous preparations, many of 

 which were made by Dr. W. B. Kirkham. 

 In the formation of the polar bodies, the 

 eggs of the rat agree closely with those 

 of the mouse, the development of which 

 have recently become well known by the 

 publication during the past year of the 

 reseai'ches of Kirkham and of Lams and 

 Doorme. As in the ease of the mouse and 

 most other mammals which have been 

 studied, the egg of the rat invariably forms 

 two polar bodies, the first of which is pro- 

 duced in the ovary, while the second is 

 formed in the Fallopian tube immediately 

 after the entrance of the spermatozoon into 

 the egg. At the time of ovulation the egg 

 has therefore already formed its first polar 

 body, and the second polar spindle has 

 become fully developed. The entrance of 



the spermatozoon stimulates the egg cell, 

 and the second polar body is immediately 

 extruded. Eggs which have formed the 

 first polar body, but which for some reason 

 fail to be discharged from the ovary at 

 the time of ovulation, degenerate and 

 eventually break up into fragments, simu- 

 lating an irregular, abnormal cleavage. 

 These degenerative changes are brought 

 about, at least in part, by the abnormal 

 activities of the previously formed second 

 polar spindle. 



The first polar spindle is easily dis- 

 tinguished from the second by the nature 

 of the chromosomes, the former containing, 

 as Kirkham has described for the mouse 

 egg, tetrads which differ considerably in 

 size, and which often split precociously 

 into dyads, while the latter contains either 

 dyads or the separated and often rodlike 

 univalent chromosomes. Both spindles 

 may show at their poles one to three or 

 more dark granules, usually of irregular 

 shape, which may be looked upon as cen- 

 trioles. 



As in the mouse, also, the second polar 

 body can usually be distinguished from the 

 first by the arrangement of the chromo- 

 somes. In the first polar body these 

 chromosomes are usually scattered irregu- 

 larly, while in the second polar body they 

 are commonly fused together into a dense 

 nuclear mass even before the complete 

 separation of the polar body from the egg 

 substance. 



The egg of the rat, furthermore, re- 

 sembles that of the mouse and differs from 

 most other mammalian eggs in that in the 

 vast majority of cases the first polar body 

 has disappeared before the egg reaches the 

 Fallopian tube. Eggs in the course of 

 fertilization and cleavage, therefore, are 

 usually accompanied by but a single polar 

 body, namely, the second. 



The entire substance of the spermatozoon 

 enters the egg, the head quickly forming 



