SIXTH LECTURE. 



THE CELL ORIGIN OF THE PROTOTROCH. 



A. U. MEAD, 

 Brown University. 



Notwithstanding the wholesome reaction against the fre- 

 quently reckless application of Baer's biogenetic law, larval and 

 embryonic characters still hold an important place as criteria of 

 the relationship of organic forms. The larval characters of the 

 tunicates are significant indices of the relationship of these 

 animals to the vertebrates, but perhaps no larval form has 

 rivalled the trochophore or trochosphere in provoking phylo- 

 genetic speculation. The splendid work of Roule, U Anatoinie 

 Comparee, which has appeared within the last year, bears wit- 

 ness to the importance which is at present attached to this 

 larval type in establishing the relationship of such various 

 forms as the annelids, rotifers, bryozoans, mollusks, etc. All 

 these forms are grouped together by him into one branch of 

 the animal kingdom, the TrocJiozoa ; for, although the bodies 

 of the different TrocJiozoa are highly variable in form and struc- 

 ture, they all may be derived in the normal embryonic develop- 

 ment from the trochophore larva, which is constructed on one 

 constant organic plan. 



The features characteristic of the trochophore arise in 

 various TrocJiozoa under very different conditions of embryonic 

 development, and are possessed by some adult animals, for 

 example, TrocJwspJicsra (Semper). These facts give deep 

 interest and wide bearing to the comparative study of the 

 embryonic origin of the trochophore itself. 



One of the essential structural characters of this larval form 



