Description of the NotocJiordal Invagination. 15 



yolk below. Only in one case did he observe a small ectodermal 

 sinking at the anterior end of the streak, and in this instance the 

 cavity was plugged with cells. Selenka ('82, p. 556) considers a 

 small ectodermal pocket in his so called gastrular region of the 

 mouse, to be a rudiment of the neurenteric canal. 



The foregoing examples merely serve to illustrate the fact that 

 the posterior dorsal opening of the notochordal canal has become 

 in Mammalia a rudimentary structure. Indeed, a few examples 

 do exist in Mammalia in which the neurenteric canal has a distinct 

 lumen, but none show a dorsal opening comparable in extent to 

 that seen in reptiles. 



In the Chrysemys embryo under consideration (Plate VIII.) 

 we see no diminution in the size of the potential canal as com- 

 pared with other embryos. The limits of the canal are not 

 determined even at this stage, for we have seen in Figure 35 

 that the lumen is still expanding on the right at the expense of 

 the cells in that region. A retardation in the development of the 

 ectodermal opening of the notochordal canal in the present 

 instance may be a forerunner of the condition found in higher 

 vertebrates, — birds and mammals,^ — a condition in which the 

 ectodermal opening develops late or not at all. The neurenteric 

 canal of birds not only develops as a very restricted lumen when 

 compared with that of reptiles, but it develops at a stage in which 

 many protovetebrae are found, Gasser describes a neurenteric 

 canal in the goose at a stage when fourteen protovertebrae are 

 present. According to Balfour ('85, p. 163) the second neuren- 

 teric canal opens in the duck at the stage with twenty-six pro- 

 tovertebrae. 



In this Chrysemys embryo it seems evident that the lumen of 

 the notochordal canal could not have been invaginated from the 

 dorsal surface. The notochord must therefore have been laid down 

 as a solid process, which is secondarily becoming hollowed out. 

 It seems interesting that even in one instance the development of 

 the notochordal canal of a reptile should proceed in a manner so 

 comparable to that which exists as the rule in mammals. 



All students of reptilian embryology, so far as I know, have 

 stated that the lumen of the notochordal canal develops from the 

 posterior toward the anterior region of the shield. Will figures 

 one instance of the gecko ('92'' , Taf. 7, Fig. 47) in which there 

 can be little doubt of an ectodermal invagination, since entoderm 



