442 



INVERTEBRATA 



CHAP. 



middle line between a x and a z ; a 2 is below and in front, whilst a : is 

 above and bebind ; the effect of this is to force apart the limbs of the 

 horse-shoe formed by the ventral cell family and to force the terminal 

 cell C backwards and downwards (Fig. 344). A new T-piece is then 



FIG. 344. -The 12-cell stage of the segmentation of the egg of Ascaris megalocephala, 

 seen from the left and the right sides respectively in order to show the rearrangement 

 of the daughters of the cell AB. (After Zur Strassen. ) 



A, from the left side. B, from the right side. The daughters of A have the lightest tone. Those of B 

 have the same tone crossed with oblique lines. The ventral cell family have the deepest tone. 



formed, which persists throughout the whole development. The cross- 

 piece of this T is horizontal and is made by the two cells a z and a z , 

 the beam is constituted by the two cells 04 and a, (Fig. 345). 



rt 



a. 



err 



y D 



FIG. 345. Dorsal view of the segmenting 

 egg of Ascaris megalocephala in the 

 20-cell stage. (After Zur Strasseu.) 



FIG. 346. Ventral view of the segmenting 

 egg of Ascaris megaloctphala in the 24- 

 cell stage. (After Zur Strassen. ) 



In this stage the embryo is an egg-shaped blastula. 



Then the cells of the ventral cell family divide. MST divides into 

 a right cell denominated mst, and a left one denominated /MTT. E 

 divides into E : in front and E 2 behind, P 3 divides into P 4 in front 

 and D behind, and C divides into a right cell c and a left cell y. 



