xv NEMATODA 445 



Next, the second pair of stomodaeal cells come into contact with 

 each other, the first pair are in contact from their first formation, 

 and this group of four sinks inwards. Of the posterior group of the 

 tail cells each divides into anterior and posterior daughters, and in 

 this way a double row of eight is formed (i.e. four pairs, one behind 

 the other). Each transverse row of four, i.e. the row formed from 

 the front tail cells and the one formed from D, folds up into a 

 U -shape. The hinder U merely continues the line of the tail cells, 

 but the front one sinks inwards and forms, as Mliller has asserted, 



U 



FIG. 349. Segmenting egg of Ascuris megalocephala in the 102-cell stage seen from 

 beneath. (After M tiller.) 



AH the ectoderm cells which are descendants of AB have a light tone the descendants of A and 

 those of B are not distinguished from one another, blp, blastopore. u, the front row of tail cells 

 descendants of C, which bend up so as to form a U. A similar U in front of this one is formed by the 

 descendants of D (dj, d2, Si, &j) and gives rise to mesoderm. 



a posterior mesoderm rudiment. (Zur Strassen had imagined that 

 it gave rise to the proctodaeum, but this is an error.) Miiller further 

 asserts that a third mesoderm rudiment is formed, at a later stage, 

 from some of the tail cells (Fig. 349). 



Zur Strassen maintains that the tail cells, the descendants of C, 

 are distinguishable from the descendants of AB, which he denominates 

 the primary ectoderm, by their clearer protoplasm and flatter shape ; 

 and he maintains that, as development proceeds, the tail cells pro- 

 gressively displace the primary ectoderm cells till these last form 

 only a small cap at the anterior end of the embryonic worm. Miiller, 

 however, has not been able to confirm this. The blastopore closes 



