REMARKS ON THE DEVELOPMENT OF AMPHIOXflS. 357 



the corresponding ectoderm ic thickening in the younger 

 larva by the growth of its base. Had an upgrowth or 

 invagination taken place, it must have pushed back the 

 pharyngeal wall, an event which Ave can see from the figures 

 has not taken place. The same conclusion will be arrived at 

 by a comparison with one another of figs. 9 and 10, which 

 represent sections through the posterior pharyngeal region 

 of two larvfB, one with free ridges and one with a rudi- 

 mentary atrial cavity. One can see that we have to do with 

 a process of elongation of the walls of the cavity, for parts in 

 fig. 10, as compared with those in 9, are stretched out. 



The farther growth of the atrial cavity is described by 

 Lankester and Willey iu terms with which I cannot agree. 

 They say that the atrium encroaches on the space hitherto 

 occupied by the coelom, and finally nearly surrounds the 

 alimentary canal. I freely admit that this is a view which is 

 at first sight strongly suggested by an examination of sections 

 through the region intervening' between the end of the 

 pharynx and the atriopore. It seems to me, however, that 

 an examination of the relations existing between the atrial 

 cavity and the gut in the pharyngeal region necessitates a 

 different view. The point of origin of the atrial wall 

 is in all stages of development situated at the edge 

 of the gill-slit. In the larva the gill-slit is a mere pore 

 situated near the ventral line, and the atrial wall appears to 

 arise in this stage near the ventral line also. As the animal 

 assumes the adult form the gill-slit elongates enormously in 

 a dorso-ventral direction, and the atrial cavity grows in strict 

 correspondence with this enlargement. The atrial cavity 

 certainly enlarges, but it does not " displace the coelom," or 

 acquire new relations ; its expansion is only a part of the 

 general expansion of all the ventral structures of the animal. 

 A comparison with one another of the pharynx of the larva 

 and adult as to histological structure confirms this view. 

 We find that the whole of the lateral walls of the pharynx of 

 the larva have the same structure as the hyperpharyngeal 

 groove and adjacent epithelium of the pharynx of the adult. 



