NOTES ET REVUE xx, 



equal size, form a Greek cross, whicli extends across IIk^ active 

 (anterior) pôle of the egg. The « cross cells » are larger than tlie 

 others just mentioned, and lie in the angles of this (iguro. The 

 intermediate cells are formed in the ;{()-48-celled stage by radial 

 cleavage, which however shows a trace of the lœotropic spiral. 



Immediately behind thèse cells, that are destined to form the 

 apical plate of the trochophore, are the sixteen large « priiuary » 

 cells of the prototroch, which form a complète girdle around the 

 egg, except that in the mid-dorsal line the cells in the two dorsal 

 quadrants come into contact merely at a point, instead of along a 

 line as is the case at the junctions of the other quadrants. 



Fig. 3. 



FiG. 3.— Cells at the active pôle in 48-celled stage of Ph. vulgare. Rosette cells dotted, 

 intermediate cells unsliaded, cross cells barred. One " primary " prototroch cell has 

 been purposely omitted. 



The cells of the posterior hémisphère at this stage are (1) the 

 daughter cells of the three ventral micromeres of the second quar- 

 tette (2 a — 2 c), which are to furnish the girdle cells that bear 

 the postoral circlet of cilia in Ph. vulgare and perhaps also the 

 three « secondary » prototroch cells, (2) the descendants of 2d, the 

 two large cells which give rise to the somatic plate (dorsal in the 

 embryo), (3) the third set of micromeres (3a — ."Ul) wliicli are to 

 form ectoderm and (4) the common mother cells of both endoderm 

 and mesoderm (3 A — 3 D). 



IV. The mesoderm arises from the dorsal représentative in the 

 48-64-celled stage of the fourth group of micromeres (viz. 4d). 



