EMBRYOLOGICAL CRITERION OF HOMOLOGY. m 



the wall of the atrial chamber, that is, from what was originally 

 ectoblast. In both cases the later history is the same.^ The 

 inner vesicle, namely, divides into three sacs, of which two 

 give rise to the atrial chamber of the bud, while the third 

 (median) forms the alimentary canal and from its dorsal wall 

 arises the ganglion. 



/ ITQavxA. \ 



1j^"5, iamWuO 



XiUOy 



DIAGRAM II. 



Bud-development contrasted with egg-development in the compound tunicates. 

 P, the inner sac ; A, atrial chamber ; g, ganglion. The figures are purely dia- 

 grammatic. 



From whatever point of view we regard this case we are 

 confronted with a fatal dilemma. Thus, in the case of Botryllus, 

 if we regard the inner and outer layers of the bud as corre- 



* See Hjort, Mitth. Zool. St. Naples, X, 1893 5 ^"d Anat. Anz., X, 1894. 



