442 



INVERTEBRATA 



CHAP. 



middle line between c and 9 ; is below and in front, whilst a x is 



9 



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 terminal 

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



Fl. 344. The 12-cell stage of the .segmentation of the egg of Afn'un'.-i iii<'<i<t/<i<-<'j>lnil<i, 

 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. 7 , 

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



err 



FIG. 345. Dorsal view of the segmenting 

 egg of Ascaris megalocephala 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 



;i right cell denominated mst, and a left one denominated 



E 



divides into E x 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. 



