442 



INVEETEBRATA 



CHAP. 



middle line between a^ and a^ ; a^ is below and in front, whilst a-^ is 

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

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

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



Fig. 344. — The 12-cell stage o( the segmentation of the egg of Ascaris inegalomphaXa, 

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

 have the same tone crosseil 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^ and a^, 

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



N 



VT 



-D 



Fig. 345. — Dorsal view of the segmenting 

 egg of Ascaris viegalocephala in the 

 20-cell stage. (After Zur Strassen.) 



Fig. 346. — Ventral view of the segmenting 

 egg of Ascaris megalocephala 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 Ej in front and E^ behind, Pg divides into P^ in front 

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



