62 C. 0. Whitmau 



Chapter III. 



The Development of the Vermiform Embryo and the Origin 

 of the (rerni-cells. 



With respect to the deavage aud the formation of the epibolic 

 Gastrula, my observations differ but little from those of Van Beneden ; 

 biit I am unable to coufirm his Statements on the origin of the germ- 

 cells and on the relation of the >^oral pole« to the blastopore. 



The first cleavageof the germ-cellalways, so far asicanjudge, results 

 in two nnequal parts, as seen in fig. 61 [D. microcephalum) and fig. 65, 

 PI. 5 [D. mo sc hat um). The sanie is true of the Orthonectidae according to 

 JuLiN. It often happens that one finds germ-cells divided intotwo equal 

 parts. bnt I believe these are all cases of multiplicative division, not of 

 cleavage. The cleavage is introdnced by the formation of a karyoly tic 

 figure 'fig. 63), in which I have been unable to recognize polar asters. I 

 have reason to believe, however, that these asters are present even where 

 they seem to bewholly wantingi. The spindle-fibres are found in the cen- 

 tral aswellas the peripheral portions of the nuclear area (Öeeoptical sec- 

 tion, fig. 64) , similarly as shown by Mark in the ^^^ of Limax (45, 

 p. 229) . The twocellsresulting from the first cleavage increase consider- 

 ably in volume before the second cleavage. and they do not divide simulta- 

 neously. I am not able to say with certainty which cell divides first, but 

 probably it is the smaller. The three-cell stage is followed by a four-cell 

 stage, in which the two pairs of cells lie et rigbt angles (figs. 69 et 70). 

 One of the four cells — the larger — soon takes the position seen in fig. 71 , 

 the other three cells formiug a sort of cap. At this stage the smallestof the 

 four cells is quite as larga as the original germ-cell. From the interest- 

 ing fact that t]ie volume increases as the cleavage advances^, we may 

 infer that considerable intervals elapse between successive cleavages. 

 The lai-ge cell takes no further part in the cleavage, but becomes grad- 

 ually enveloped as the uumber of cells in the ectoderma! cap in- 

 creases. 



An optical section of the eleven-cell stage (fig. 74) shows the large 

 central cell inclosed in an ectodermal mantle, which is open only on one 

 side. Figs. 76 and 77 represent an optical section and a surfaceview of 



1 In the gcrm-band period of Clepsine , I find many small cells in which the 

 karyolytic figures, seen from one side, show only the spindle; in others, where the 

 figurs are cut obliquely, the asters are distinctly visible. 



~ JuLiN has observed the saiue in the Orthonectidae. 



