72 Jenkinson, Gerviinal Layers of Vertebrates. 



fortunately it is possible, in the 'cell-lineages ' of many 

 Molluscs, Annelids, and Turbellarians, by tracing back 

 an organ or layer to a single cell to establish its origin 

 beyond cavil or question. It will be sufficient for the 

 purposes of this argument to cite the most striking 

 discrepancies. 



In the three groups just mentioned the segmentation 

 of the &%% proceeds upon a very regular so-called ' spiral ' 

 plan {Fig. 34). The first two divisions — meridional, 

 and at right angles to one another — produce four cells 



Fig. 34. 

 Diagram of the formation of the first (la — \d), second {za — 2d), and 

 third (3a — yl) quartettes of micromeres from the macromeres {A — D) in the 

 egg of a Turbellarian, Mollusc, or Annelid. 



lying, roughly speaking, in one plane. Of these four cells, 

 one is left, one is anterior, one right, and one posterior ; 

 they are known as the macromeres, and termed respectively 

 A, B, C, and D. Each macromere now buds off towards 

 the animal pole, and in a right-handed direction a small 

 cell or micromere ; the four constitute the first quartette, 

 and are termed (following Conklin's notation) \a, lb, \c, 

 and id. A second quartette {2a, etc.) is given off left- 



