300 



EMBRYOLOGY 



Rq. 7. <* 



FIG. 7. The female aua male chromosomes of Fig. 6 have divided longitudinally 

 and separated from each other in two groups of daughter chromosomes: (sp) 

 spindle; (c) centrosome. 



FIG. 8. The two halves of the egg contain daughter nuclei, half of whose chromo- 

 somes arise from the egg-nucleus, half from the sperm-nucleus. 



been constantly increasing by growth. Of course the equal division 

 cannot be determined later by direct observation, as is the case in 

 the first division, but after what we know of the nature of nuclear 

 division this view may be considered in the highest degree probable. 



Still more important is the second fact determined on the Ascaris 

 egg. The chromosomes of the egg- and sperm-nuclei are an excep- 

 tion to the above-mentioned numerical law. Whereas in Ascaris 

 megalocephala bivalens four chromosomes always arise from the 

 resting nucleus, only half as many, that is, two, occur in the egg- and 

 in the sperm-nuclei (Figs. 4, 5, eik and sk). How is this exception 

 from the numerical law to be explained? How is it brought about? 

 A very accurate study of the method of origin of the polar cells, 

 as is possible in Ascaris, gives a satisfactory explanation. 



Some time before the origin of the polar cells remarkable changes 

 occur in the contents of the nucleus which justify the great con- 

 sideration which they have received, and which have been the 

 object of extended investigation. In this variety of Ascaris four 

 long threads arise from the chromatin network and split longitud- 

 inally into double threads before the height of karyokinesis, the 

 usual time of splitting. These threads immediately place themselves 

 across each other, and thus produce, while gradually becoming 

 shorter, a tetrad of chromosomes, a stage which has been shown 

 in the development of many species of animals. When now the 

 nucleus dissolves and the first polar spindle is formed from its 

 contents, the eight chromosomes arrange themselves in the middle, 

 in two tetrad groups. Later each tetrad group, of the first polar 

 spindle, separates into two groups of chromosomes, connected in 

 pairs (Fig. 3) , or in other words, each tetrad divides in two dyads, 

 of which one passes into the first polar cell (pzT), the other passes 

 into the egg. 



