274 REDUCTION OF THE CHROMOSOMES 



pairs." ^ In this case their number seems to be at first the somatic 

 number (thirty-six), which is afterward halved by conjugation of the 

 elements two and two (Riickert), as in Ljimbj^cus (Calkins). It is, 

 however, certain that in many cases (insects, copepods) the double 

 fods first appear in the reduced number, and the observations of Vom 

 Rath ('93) and Hacker ('95, 3) give some reason to believe that the 

 reduced number may in some forms be present in the earlier progeni- 

 tors of the germ-cells, the former author having found but half the 

 normal number in some of the embryonic cells of the salamander, while 

 Hacker ('95, 3) finds that in Cyclops brcvicoriiis the reduced number 

 of chromosomes (twelve) appears in the primordial germ-cells which 

 are differentiated in the blastula-stage (Fig. 74). He adds the inter- 

 esting discovery that in this form the somatic nuclei of the cleavage- 

 stages show the same number, and hence concludes that all the 

 chromosomes of these stages are bivalent. As development proceeds, 

 the germ-cells retain this character, while the somatic cells acquire 

 the usual number (twenty-four) — a process which, if the conception 

 of bivalent chromosomes be valid, must consist in the division of each 

 bivalent rod into its two elements. We have here a wholly new light 

 on the historical origin of reduction ; for the pseudo-reduction of the 

 germ-nuclei seems to be in this case a persistence of the embryonic 

 condition, and we may therefore hope for a future explanation of the 

 process by which it has in other cases been deferred until the penul- 

 timate cell-generation, as is certainly the fact in Ascaris? 



This leads to the consideration of some very interesting recent dis- 

 coveries regarding the relation of reduction to the alternation of gen- 

 erations in the higher plants. As already stated (p. 263), Strasburger, 

 Guignard, and other observers have found that in the angiosperms 

 the two maturation-divisions are in both sexes followed by one or 

 more divisions in which the reduced number persists. The cells thus 

 formed are generally recognized as belonging to the vestiges of the 

 sexual generation (prothallium) of the higher cryptogams, the pollen- 

 grains (or their analogues in the female) corresponding to the asexual 

 spores of the archegoniate cryptogams. We should, therefore, expect 

 to find reduction in the latter forms occurring in the two correspond- 

 ing divisions, by which the " tetrad " of spores is formed (as was first 

 pointed out by Hartog, '91). Botanists were thus led to the surmise, 

 first expressed by Overton in 1892, that the reduced number would 

 be found to occur in the prothallium-cells derived from those spores. 



1 '92, 2, p. 51. 



- It may be recalled that in Ascaris Boveri proved that the primordial germ-cells have 

 the full number of chromosomes, and Hertwig clearly showed that this number is retained 

 up to the last division of the spermatogonia. Ishikavva ('97) finds that in Allium the 

 reduced number (eight) appears in the mitosis of the " Urpollenzellen " preceding the 

 pollen-mother-cells. This is, however, contradicted by Mottier ('97, 2). 



