328 McMURRICH. [Vol. IV. 



have had an independent origin from the median streak. If 

 we may judge from the individual development, the median 

 streak was the first to make its appearance in the ancestors of 

 the Actinozoa, and they remained with such simple glandular 

 filaments, formed by a differentiation of the endodermal cells 

 along the free edges of the mesenteries, for some time. Later, 

 the ciliated respiratory portions of the mesentery developed by 

 a down-growth of the stomatodasal ectoderm, and were at first 

 confined to the upper portion of the mesentery where the gland- 

 ular streak was not developed, but later, extending their limits, 

 they came to overlap the glandular portion, and thus the fila- 

 ment became trilobed throughout a portion of its length. 



Clark University, Worcester, Mass., 

 October 14, 1890. 



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