DEVELOPMENT OF PETROMYZON FLUVIATILIS. 179 



is a section taken through the blastopore and the head soon 

 after the head is raised above the general level of the egg. 



From his observations of the embryo as a whole, Schultze 

 came to the conclusion that the blastopore persisted and gave 

 rise to the anus, and he was supported in this view by Calberla. 

 Later observers, however, who have studied the development 

 of the Lamprey by means of sections, have maintained 

 Benecke's view that the blastopore disappears. Scott 

 describes the neural canal enclosing the blastopore and figures 

 the neurenteric canal thus formed. He describes the formation 

 of the anus, from a protuberance of the alimentary canal 

 which approaches the epidermis and breaks through about the 

 twentieth day. Balfour also states that the blastopore closes 

 and does not form the permanent anus. 



My observations of the embryo as an opaque object lead me 

 to the belief that the blastopore remained open. In this I 

 have been confirmed by sections taken through a series of 

 embryos preserved at intervals of a few hours. Primarily the 

 blastopore lies at the posterior dorsal end of the embryo 

 (fig. 4), but by the growth of the dorsal surface and the forma- 

 tion of the tail it comes to occupy a position in the ventral 

 surface. What was the anterior lip in the first position comes 

 to be the posterior in the latter. 



Fig. 4 is a view of the embryo twelve days old, as an opaque 

 object, showing the blastopore at the posterior end of the 

 neural ridge. Fig. 16 is an oblique section through an embryo 

 about two days older, showing the nervous cord just separated 

 from the skin, and the notochord both continuing behind the 

 blastopore. 



Scott was of opinion that the lumen of the invaginated 

 mesenteron persisted only in the fore-gut. Soon after the in- 

 vagination is completed this part of the alimentary canal lying 

 in the head and neck becomes raised from the rest of the 

 embryo. It is thus separated off from the yolk-cells, and the 

 hypoblastic cells in this region soon assume a definite columnar 

 appearance, though they continue to contain yolk granules for 

 some days. This region extends to where the liver appears 



