352 E. W. .MACIUIIDK. 



lu the same number of this Journal in which my paper 

 appeared, Professor Lankester published a criticism (4) of 

 some points contained in it. He considered that in referring 

 to a paper by himself and Willey (3) I had given the impres- 

 sion that they had not deviated from Kowalevsky's view on 

 the subject of the development of the atrial cavity ; and that 

 further, since in my own paper I used the term "■' atrial fold " 

 to denote the wall of the atrial cavity, I had virtually adopted 

 Kowalevsky's view on this subject, and overlooked the cor- 

 rection of this view by Dr. Willey himself, and that in so 

 doing I was '' perpetuating error," 



Such a strong statement from a zoologist occupying the 

 position of Professor Lankester could not be passed over in 

 silence. But various preoccupations, especially those con- 

 nected with the organisation of the Zoological Department in 

 McGill University, prevented my undertaking a renewed 

 examination of my preparations before now. This I have, 

 however, at last accomplished, and the results of this exami- 

 nation are given in the present paper. In general it may be 

 said that these results confirm Kowalevsky's position as to 

 the mode of formation of the atrium, but in many details 

 they support the observations of Lankester and Willey. 



Kowalevsky observed (2) in the older larvae of Amphioxus 

 that two ridges appeared on the ventral surface. Subse- 

 quently he found underneath the animal a median tube — the 

 rudiment of the atrial cavity. This tube was situated be- 

 tween the ridges observed in an earlier state, and these ridges 

 could still be observed projecting from its walls. These 

 freely projecting portions could be traced into still older 

 larvEe, and Avere found to constitute the metapleural folds of 

 the adult. The canals contained in the folds were stated by 

 Kowalevsky to be extensions of the general coelom. The 

 interpretation placed by Kowalevsky on his observations was 

 that the ridges had met beneath the ventral surface of the 

 body, and so enclosed the atrial cavity. 



Lankester and Willey (3) have made similar observations, 

 but have interpreted them rather differently. Tliey say the 



