DIVERGENT ACCOUXTS OF REDUCTION 20I 



chromosomes of the normal number that have spHt longitudinally, 



a b , , r 1 r , . a b a x „ , 



I.e. T» etc., and the lormula tor a tetrad is j- or Such 



a b a b a X 



a tetrad, therefore, agrees as to its composition with the formulas of 

 Hacker, vom Rath, and Ruckert, and agrees in mode of origin with 

 the process described by Ruckert in the eggs of Pristiiirus. While 

 these observations are not absolutely conclusive, they nevertheless 

 rest on strong evidence, and they do not stand in actual contradiction 

 of what is known in the copepods and vertebrates. The possibility 

 of such a mode of origin in other forms must, I think, be held open. 



Under the same category must be placed Korschelt's unique 

 results in the egg-reduction of the annelid OpJiryotrocJia ('95), which 

 are very difficult to reconcile with anything known in other forms. 

 The typical somatic number of chromosomes is here four. The same 

 minibcr of chromosomes appear in the germinal vesicle (Fig. 96, D). 

 They are at first single, then double by a longitudinal split, but after- 

 wards single again by a reunion of the halves. The four chromo- 

 somes group themselves in a single tetrad, two passing into the first 

 polar-body, while two remain in the O-^Z, but meanwhile each of them 

 again splits into two. Of the four chromosomes thus left in the (t^^, 

 two are passed out into the second polar body, while the two remain- 

 ing in the Q'gg give rise to the germ-nucleus. From this it follows 

 that the formation of the first polar body is a reducing division (!) 

 — a result which agrees with the earlier conclusions of Henklng on 

 PyiTochoris, but differs entirely from those of Ruckert, Hacker, and 

 vom Rath. The meaning of this remarkable result cannot here be 

 discussed. A clue to its interpretation is perhaps given by Hacker's 

 interesting observations on the two modes of maturation in Cautho- 

 caniptns, for which the reader is referred to Hacker's paper ('95, i). 



Moore ('95) has given an account of reduction in the spermatogen- 

 esis of mammals and elasmobranchs which differs widely in many 

 respects from those of all other observers. In both cases there is 

 said to be a resting stage between the two spermatocyte-divisions, 

 and in mammals (rat) the reduced number of chromosomes first 

 appears in the prophase of the last division. In elasmobranchs both 

 spermatocyte-divisions are of the heterotypical form, with ring- 

 shaped chromosomes. On all these points Moore's account contra- 

 dicts those of all other investigators of reduction in the animals, 

 and he is further in contradiction with Ruckert on the number 

 of chromosomes. His general interpretation accords with that of 

 Brauer and Strasburger, reducing divisions being totally denied. 

 The evidence on which this interpretation rests will be found in his 

 original papers. 



