2/6 



REDUCTION OF THE CHROMOSOMES 



characterized, especially in the male, by a more or less complete 

 concentration of the chromatin-substance at one side of the nucleus. 

 This sta<;e, to which Moore has given the name synapsis {Y\g. 120, A\ 

 sometimes occurs when the spireme thread is already split {Ascaris, 

 Lilium), sometimes before the division is visible (insects). In cither 

 case till- cluv}natin-sig)iu)its ijfii?-i^c from the synapsis stage longitndi- 

 nallv divided and in the redneed nnniber, a fact which gives ground 

 for the conclusion that the synapsis is in some way concerned with 

 the rearrangement of the chromatin-substance involved in the numer- 

 ical reduction. During the synapsis the nucleolus remains cjuite 

 distinct from the chromatin, and in many cases it afterward persists 

 beside the tetrads, in the formation of which it takes no j^art, to be 

 cast out into the cytoplasm (Fig. 124) or to degenerate /// situ during 

 the hrst maturation-division. 



A suggestive phenomena, described by several observers,^ is the 

 casting out of a large part of the nuclear reticulum of the germinal 



ifc»-«y *•' 



A 





io-;yji.iJ-r.- 



B 



C 



Fig. 137. — Types of maturation-spindles in the female. 



./. I'irst polar spindle with tetrads, in Hetetocope. [HaCKER]. B. Second polar spindle 

 in Triton. [Caknoy and LeBkun.] C. First polar spindle of ^jrjr/^. [FiJRST.] 



vesicle at the time the polar bodies are formed (Figs. 97, 128). In 

 these cases {Asterias, PolyeJio^nis, Tlialassenia, Nereis) only a small 

 fraction of the chromatin-substance is preserved to form the chromo- 

 somes, the remainder degenerating in the cytoplasm.- 



As a final point we must briefly consider the varying accounts of 

 the achromatic maturation-figures in the female already briefly referred 

 to at page 85. In many forms {^e.g. in turbellarians, nemertines, anne- 

 lids, mollusks, echinoderms) the polar am]:)hiasters are of quite ty])ical 

 form, with large asters and distinct centrosomes nearly similar to those 

 of the cleavage-figures. In others, however (nematodes, arthropods, 

 tunicates, vertebrates), the polar spindles differ markedly from those 

 of the cleavage-figures, being described by many authors as entirely 

 devoid of asters and even in some cases of centrosomes (Fig. 137). 



1 Cf. Mathews (Wilson and Mathews, '95), Gardiner ('98), Griffin ('99). 



2 Cf. the enormous reduction of the chromatin-substance in the elasmobranch egg, p. 338. 



