THE EARLY DEVELOPMENT OF AMPHIOXUS. 591 



along the mid-dorsal Hue, or, as it was sometimes expressed, 

 the meeting of the lips of a long slit-like blastopore. 



Hatschek's account of the origin of the mesodermal struc- 

 tures is curious and interesting. Like Kowalevsky, he finds 

 two longitudinal folds of the gut wall, but these he believes to 

 terminate posteriorly in two large " pole" cells situated in the 

 lip of the blastopore. By cross-folds the successive pouches 

 are cut off from these longitudinal folds. Later two inde- 

 pendent outgrowths from the alimentary canal form the 

 " head cavities," of which the right retains longest its com- 

 munication with the alimentary canal. The formation of a 

 continuous ventral body-cavity by the fusion of the lower 

 parts of the pouches (somites) is deduced from the fact that 

 the divisions between the somites can no longer be traced in 

 this part of the body. In a later paper (4) he describes in the 

 larva what he terms "a genuine kidney/' a tube lying in front 

 of the mouth. This structure will be referred to throughout 

 this paper as " Hatschek's nephridiura." In his last paper 

 (5) he gives an account of the derivation of the muscular and 

 skeletal tissue from the coelomic pouches, pointing out that 

 everything in Amphioxus is essentially epithelial in nature, 

 the connective tissue having no nuclei in it. 



Lankester and Willey's paper on the development of the 

 atrial chamber (9) confirms in most points Kowalevsky's 

 statements, but the authors deny that the cavities in the 

 raetapleural folds are coelomic in nature as Kowalevsky had 

 imagined. The right atrial fold is described as extending 

 much further forward than the left. They found also that 

 Hatschek's nephridium opens into the gut ; a result which 

 Van Wijhe (11) announced as an independent discovery three 

 years later. 



Willey (13) in 1891 published an account of the develop- 

 ment of the gill-slits and other organs appearing in the larval 

 stage. He describes the internal and external openings of the 

 club-shaped gland (an organ noticed already by Kowalevsky), 

 which he regards as the fellow of the first gill-slit. He states 

 that the oral hood which conceals the true mouth of Amphi- 



