404 WILSON. [Vol. VI. 



Before leaving the head-kidney I may add that the vacuola- 

 tion and granulation of the prototrochal cells suggest that they, 

 too, may possess an excretory function, as has been pointed out 

 by Hatschek in the case of Polygordius (No. lo). The marked 

 tendency to vacuolation in the post-trochal cells (unfortunately 

 I am not able to state whether the vacuoles exist in the livinsf 

 embryo) suggests a comparison with the so-called external or 

 post-velar excretory cells of the molluscan veliger, with which 

 they agree in position. In the Mollusca the cells of the velum 

 itself are often extensively vacuolated, and it has been suggested 

 (for a review of the literature see McMtirrich, No. 17) that the 

 velar cells, the post-velar cells, and the head-kidneys originally 

 formed part of one system concerned especially with the excre- 

 tion of waste matters produced by the intense activity of the 

 velar cilia. In the Mollusca the post-velar excretory cells and 

 the head-kidney replace one another {i.e. do not occur together). 

 In Nereis, if the comparison has any value, the three coexist 

 side by side, and possibly represent the primitive condition of 

 the trochophore larva. 



VI. History of the Somatoblasts. Origin of the Ven- 

 tral Plate and of the Mesoblast-bands. Closure 

 OF THE Blastopore. Development of the Stomo- 

 D^UM. (Plates XVII and XVIII.) 



We turn now to the history of the lower hemisphere, in the 

 course of which the most complicated and interesting events of 

 the ontogeny take place. These events are so intimately bound 

 together that it will be necessary to give first a brief resume. 



The boundary between the upper and lower hemispheres is 

 formed by the prototroch which, as has been shown, is a prod- 

 uct of the first group of micromeres. The superficial cells of 

 the lower hemisphere are therefore wholly derived from the 

 second and third sets of micromeres and from the products 

 of the second somatoblast. 



After the formation of the second somatoblast {i.e. at the end 

 of the spiral period, Figs. 29, 31, 33, 49) there are in all fourteen 

 of these cells, viz. : — 



The three smaller secondary micromeres and their progeny 

 («2-i, «2.2^ ^2.1^ ^2.2^ ^2.1^ ^2.2^_ 



