OOGENESIS AND EARLY EMBRYOLOGY ASCARIS 565 



rest of the chromosome, one-half to two-thirds of its total volume, 

 becomes massed with corresponding parts of other chromosomes 

 into a few larger clumps, which occupy the periphery of the 

 equatorial plate. The small granules, or karyosomes, — sixty to 

 seventy-two in A. canis, — are arranged in the middle of the 

 plate. As division proceeds each of the centrally placed karyo- 

 somes divides into equal parts, and the two daughter plates, 

 consisting of these halves, migrate toward the poles of the spin- 

 dle (figs. 70, 72, 73, 75, 77), leaving the large clumps of useless (?) 

 chromatin behind. As the new cell wall forms, these clumps of 

 chromatin divide in a plane coinciding with that of the chromo- 

 somal division and go, half to each daughter cell, where they are 

 seen lying in the cytoplasm close to the newly formed cell wall. 

 In this position they undergo dissolution without further 

 activities. 



As already shown, in A. canis the larger (soma) cell divides 

 first. In half of the cases the division is one of 'diminution,' 

 various stages of which are illustrated semi-diagrammatically 

 from actual cells in figures 69, 70 and 72. In the other fifty 

 percent of the cases, the division is by ordinary mitosis (fig. 71), 

 in which the same number of dyad chromosomes occurs as in the 

 stem sells. The stem cell, dividing in a plane at right angles to 

 that of the soma cell (fig. 73), gives rise to the vertical part of 

 the ' T-shaped ' embryo (fig. 74) , which is peculiar to the nema- 

 todes. The division of the stem cell is here somewhat precocious 

 (fig. 73), as the soma cell is ordinarily completely divided before 

 the spindle of the stem cell is set up. The embryo pictured in 

 figure 74 is one in which the 'diminution' process has been de- 

 layed, and hence all four nuclei are alike. The cell at the end 

 of the upright of the ' T ' is the stem cell of the second generation. 



The stem cell (strp.) rotates upon the second soma cell in such 

 a manner that it comes to lie in the angle between the second 

 soma cell and one of the daughter cells of the first soma cell, 

 thus forming a typical lozenge-shaped embryo (side view, fig. 75 ; 

 face view, fig. 76). Figures 75 to 77 represent the condition in 

 passing from the four- to the six-cell stage, the soma cells 

 dividing before the stem cells. In all three cases figured, the 



