THE EARLY DEVELOPMENT OF AMPHIOXUS. 601 



further back, we see them quite free from the gut, and so we 

 can conclude that they are recurved. Hatschek (3) spoke of 

 the two head cavities as distinct outgrowths from the gut, and 

 further stated that the right one still communicated with the 

 gut after the left had been cut off. This is not a correct 

 account of what happens : the whole anterior part of the 

 alimentary canal becomes shut off from the hinder part, and its 

 two horns, which later become converted into the head cavities, 

 still communicate with one another after separation from the 

 gut has taken place (fig. 18, b). 



We have thus seen that the mesoderm originates as five 

 hollow outgrowths from the gut — an anterior median, viz. the 

 head cavity rudiment, and two pairs of lateral ones, viz. the 

 collar cavities and the coelomic grooves. Thus in the formation 

 of the primary layers Amphioxus is in fundamental agreement 

 with Balanoglossus, as described by Bateson (1). The main 

 difference between the two types is that whereas in Balano- 

 glossus the trunk cavity remains undivided, in Amphioxus it 

 becomes broken up into a series of segments, a difference 

 which we may plausibly correlate with the different modes of 

 life pursued by the two animals. PI. 45, figs. 25 and 26, are 

 two diagrams intended to make these relations clearer. 



3. The Fate of the Coelomic Cavities. 

 The two horns of the common head cavity rudiments rapidly 

 become separated from one another ; the right now shows 

 itself as an irregular-shaped and thin-walled sac; the left, on 

 the other hand, is composed of cylindrical cells, and remains 

 small and round (fig. 18, a). The right soon after gets shifted 

 ventrally, and forms the greater part of the cavity of the 

 praeoral snout during the whole of the larval period (compare 

 fig. 21, a). I have not been able to identify it in the adult, 

 and can only suppose that it becomes obliterated, the space 

 corresponding to it being apparently occupied by connective 

 tissue. The left, as is well known, acquires an opening to the 

 exterior, and constitutes the prseoral ciliated pit (fig. 21, a), 

 which Hatschek first discovered. This prseoral pit persists into 



