DEVELOPMENT OF PARAVORTEX GEMELLIPARA 501 



of this paper would indicate, on the other hand, that, since in 

 both embryos the micromere contains an amoeboid nucleus 

 while the macromere shows a spindle, the latter is about to 

 divide before the first undergoes division. The second cleavage 

 of the large macromere, judging from the conditions represented 

 in text figure 11, results in the formation of two smaller macro- 

 meres somewhat unequal in size. The positions of the spindles 

 which are inaugurating a further division of the two macro- 

 meres of both embryos indicate that the cleavage is to be of 

 the spiral type described by Bresslau for Mesosotomum ehren- 

 bergi. He found, however, that there were three micromeres 

 present before the second macromere was cut off. Three of 

 the macromere spindles in this pair of embryos are in the pro- 

 phase, and clearly show eight chromosomes being drawn into 

 the equatorial plate. On account of an artifact in one sec- 

 tion of the fourth macromere the arrangement of the chromo- 

 somes could not be determined. 



It is important to notice that during early cleavage the mito- 

 chondrial mass lies apparently inert at one side of the macromere 

 and entirely outside the spindle. Not until later does the mass 

 itself become divided. 



Hallez determined the number of chromosomes present in 

 the equatorial plate of the cleavage spindle of P. cardii to be 

 "four (= 4 groups binaires)." In P. gemellipara, however, 

 the equatorial plate always contains eight U-shaped chromo- 

 somes; in the telophase each pole of the spindle likewise holds 

 eight (text figs. 13, 14). 



Beyond the four-celled stage cleavage proceeds irregularly 

 as observed by Bresslau, Hallez and others. One can deter- 

 mine, however, that at one pole of the embryo the micromeres 

 become more and more numerous; two macromeres occupy 

 the other pole. 



While the macromeres give rise by division to some of the 

 larger micromeres, the latter redivide again and again. 



The embryos represented in text figures 13 and 14 contained 

 respectively fifteen and seventeen blastomeres. Text figure 

 14 is the more normal in that the group of cells is compact with- 



