NOTES ET REVUE xxiii 



synchronously in corresponding cells of llie two sides, eacli lyinK 

 at right angles to the chief axis of the embryo and parallcl lo l lu- 

 su rf ace. 



The stomadaeum is forined before elongation begins (24 hours) 

 by an invagination of ectoderm immediately in front of the région 

 of the blastopore, which bas been closed by the growth of the 



Fig. r,. 



FiG. 5.- Ventral surface of trochophore of P//. vulgare. Cilia of prototrorli are sli..\vn 

 merely on its edge. (la, cuticula. 



somatic plate ventrad. The proctodseum is established later (al the 

 âge of about 40 hours) by a slight invagination in tlie michile of llie 

 dorsal side of the trunk, which coiaes into contact with a sohd 

 mass of endoderm. 



The ventral nerve cord appears in the trociiopliore as an 

 unpaired thickening of the ectoderm along the niid-ventral line. 

 Outgrowths extend forward from its anterior extremity on each 

 side of tlie stomadaeum, and unité with thaï part of Ihe apical plafe 



