98 PRINCIPLES OF EMBRYOLOGY 



In many of the spirally cleaving eggs, the determination of future 

 development is complete (or nearly complete) even earlier than in 

 Cerebratulus. We have seen that the id and 4^ cells are particularly im- 

 portant especially in species with no larval stage; and it is often found 

 that the D quadrant is already different from the others from the very 



Figure 6.3 



Mosaic development in the nemertean worm Cerebratulus. A and B show the 

 normal 8- and i6-cell stages. C is the normal 'pilidium' larva (a variety of 

 the usual trochophore larva). D is a relatively normal pihdium from an 

 isolated blastomere from the 2-cell stage. E, the same from an isolated blasto- 

 raere of the 4-cell stage. F, larva from isolated four animal cells from 8- 

 cell stage; note the absence of any gut and the exaggeration of the apical 

 tuft. G, larva from four vegetative cells isolated at 8-cell stage. H, middle 

 two layers (ang and vegj) isolated from i6-cell stage. /, larva from combined 

 aUj and vegg from i6-cell stage. Note the failure of regulation in H or of 

 interaction in /. (From Horstadius 1937.) 



beginning of development. Thus in the oligochaete Tubifex (Review: 

 Lehmann 1948^7) only the cell containing the D quadrant will develop any 

 embryonic structures if the first two or the first four blastomeres are 

 isolated. There is therefore already something in the D quadrant which is 

 necessary for the formation of an embryo. But this does not mean that 

 all the details of development have been completely fixed by this time. 



