The Development of Ischnochiton. 587 



of 3d^ becomes lengthened. Later a perfectly symmetrical condition 

 arises which continues henceforth. 



In this case it is most obvious that the mesoblast is modified 

 from the entoderm. It arises a little earlier than the other members 

 of the fourth quartette and is comparatively larger, but the general 

 features of its origin and position are unmistakable. 



Mead urges that the mesoblast arises at the ideal 64 cell stage, 

 and while this ap})ears to be correct many modifications exist which 

 destroy this typical condition. In such forms as Crepidula the ecto- 

 blast is slow in its development and the mesoblast arises at the 

 25 cell stage, and all conditions exist between this extreme and 

 Ischnochiton where the mesoblast arises as the 73r(l cell. These 

 modifications have probably arisen from various causes, such as the 

 accumulation of yolk and precocious segregation, but the most re- 

 markable fact of it all is that despite these acquired characters the 

 mesoblast in Annelids and Molluscs arises at the fourth 

 division of the posterior macromere. 



In this connection it is important to note that in Ischnochiton 

 there is another addition to the rapidly increasing list of forms in 

 which the main structural details of development are similar. Among 

 Annelids, flatworms and Molluscs, save Cephalopods, the first three 

 quartettes of cells give rise to the entire future ectoblast, and at the 

 next division of the left posterior macromere the primary mesoblast 

 arises, while the remaining cells of the fourth quartette and the 

 macromeres form entoblast. And these remarkable resemblances do 

 not cease at this point but continue more or less clearly defined as 

 far as the development may be followed. Their fuller significance 

 therefore will be considered when these later phases have been de- 

 scribed and the evidence is all in. 



Also the form of the future embryo and larva are at this time 

 outlined, the protoblasts of all the more important regions and organs 

 of the body being present. This, is shown in the following table which 

 is here introduced that it may render more intelligable the relation 

 of the various quartettes and the part they play in the development. 



^ V. History of the First Quartette. 



As will be remembereil the first quartette of ectomeres gives rise 

 to the head vesicle, the apical sense organ , cerebral ganglia , and a 

 portion of the prototroch. There are no head kidneys. 



