1885.] 



and Blastopore of the Lamprey. 



conclusion that blastopore persisted as the anus, and this view was 

 supported by Calberla. On the other hand the later observers, who 

 have studied the development by means of sections, have maintained 

 Benecke's view that the blastopore closes. Scott describes the neural 

 canal as enclosing the blastopore, and by its closure forming a 

 nenrenteric canal, which he figures. He states that the anus is sub- 

 sequently formed by a protrusion of the alimentary canal against the 

 skin, which becomes open about the twentieth day. Balfour states 

 that the blastopore closes, and does not form the permanent 

 anus. 



My observations on the embryo as an> opaque object led me to the 

 belief thai the blastopore remained open, and in this T have been 

 confirmed by a number of series of sections taken from embryos of all 

 stages, from the commencement of the invagination until the time 

 when the cloaca is definitely established. At its first appearance the 

 blastopore is situated at the posterior dorsal surface of the embryo, 

 but by the elongation of the embryo and the formation of the tail the 

 blastopore comes to occupy a position on the ventral surface. 



Scott was of opinion that the lumen of the invaginated mesenteron 

 persisted only in the fore-gut. This part of the alimentary canal 

 shortly after the invagination is- completed is raised with the head 

 from the rest of the- embryo.. This part is therefore free from the 

 large yolk cells, and the cells lining the mesenteron soon assume a 

 definite columnar character, although they continue to contain yolk 

 granules for a considerable time. According to Scott and Calberla the 

 lumen of the mesenteron isn the trunk entirely disappears, and only 

 appears again at a muchi later stage*. My sections, however, show 

 that the lumen never really disappears. At its anterior end, as is just 

 mentioned, its lining; cells soon become columnar, and these extend 

 from its blind anterior' end to the posterior part of what will subse- 

 quently form the gill region.. A similar change takes place at 

 the posterior end. The cells surrounding the blastopore, and extend- 

 ing for some distance into the alimentary canal, very early assume a 

 columnar character,, and are in fact indistinguishable from the epider- 

 mal cells. The cells lining the- mid-gut do not assume this epithelial-like 

 character till a much later stage. The dorsal row are, however, more 

 columnar than those on the ventral side ;. these latter have just the 

 same characters as the other yolk cells. 



At its posterior end the neural tube becomes solid, and this solid 

 rod soon fuses with the posterior end of the notochord, and with a 

 solid rod of cells which pass backwards from the hind-gut, and pro- 

 bably represent the post-anal gut. A little further back the meso- 

 blastic plates join this mass of indifferent tissue ; so that we have behind 

 the anus, in a position corresponding to the front lip of the blastopore, 

 when it occupied its primitive position on the posterior dorsal surface, 



