No. I.] 777^ EMBRYOLOGY OF CREPIDULA. 151 



served much longer. The asymmetrical origin of the larval 

 mesoblast in Unio is probably associated with the extreme pre- 

 cocity of development which is shown in the very early differ- 

 entiation of this and of so many other fundaments in that 

 animal. In all cases which I have observed, the larval meso- 

 blast cell in quadrant A, which is the only one found in Unio, 

 is the first one formed in Crepidula. 



It is a most interesting fact that the larval mesoblast in 

 Crepidula arises in the three quadrants which have produced 

 no other mesoblast, viz., A, B, and C ; the quadrant D, which 

 gives rise to the paired mesoblast, produces no larval meso- 

 blast.i 



Mesoblast is therefore produced in each of the four quad- 

 rants. In A, B, and C it is derived from the ectomeres of the 

 second quartette ; in D, from the fourth quartette. In all 

 cases the segregation of mesoblast in the cell 4d is associated 

 with elongation of the body and teloblastic growth, even in 

 such animals as lamellibranchs and gasteropods, which are not 

 generally considered elongated animals. In more primitive 

 forms there is probably no teloblastic growth, and consequently 

 the mesoblast may arise in the same way in each quadrant, as 

 is said to be the case among polyclades and ctenophores. 



From these facts it is probable that the radial origin of meso- 

 blast is to be considered a primitive character; its bilateral 

 origin, a secondary one. In other words, the larval mesoblast 

 is the more ancestral, and it might properly be called the 

 primary or 7'adial mesoblast, while that formed from 4d might 

 be known as secondary or bilateral inesoblast. 



Throughout embryonic life all the mesoblast is but scantily 

 developed and exists for the most part as fusiform cells. These 

 cells are most numerous in the foot, where they form the myo- 

 cytes which traverse the cavity of the foot in every direction ; 

 they are also found in considerable numbers in the velar lobes 

 and in the head vesicle. 



1 Of course it is possible that this quadrant does give rise to larval mesoblast 

 at a stage much later than that at which it arises in the other quadrants. The 

 most diligent search, however, has so far failed to reveal it, and if larval meso- 

 blast is ever produced in quadrant D, it must be so much later than its origin in 

 the other quadrants as to deserve to belong to another series of phenomena. 



