Description of the NotocJiordal Invagination. 1 1 



odorata show that the lumen of the notochordal canal continues 

 forward for three or four sections in front of the anterior margin of 

 the break, and then ends blindly. A cross section of the lumen 

 in this anterior region is represented for Ozotheca in Figure 57 

 (Plate XL). The Chrysemys embryo shows in cross sections of 

 this region the same condition. From the blind end of the lumen, 

 in both species, a solid layer of tissue extends forward under the 

 remainder of the shield. In these three genera, namely, Chelydra, 

 Chrysemys, and Ozotheca, the invagination cavity at the time of its 

 union with the subgerminal cavity is not always coextensive with 

 the area of the shield. The original figures of Agassiz ('57), and 

 Will's reproduction of them, show the lateral extension of the 

 invagination cavity to be less than that of the shield. 



The statement quoted above from Will, to the effect that the 

 invagination is hollow throughout its whole extent, leads us now to 

 the question concerning the method in which the lumen of the in- 

 vagination is formed. Referring again to Plate IV. Fig. 14, the 

 walls of the notochordal cavity of individual yS, as has previously 

 been noted, are lined with cornified, degenerating cells. The apex 

 or anterior end of this lumen is likewise filled with these same dis- 

 integrating cells. Figure 52 (Plate X.) shows the floor of the 

 notochordal cavity of another embryo, — Chrysemys picta, — 

 which is lined by the same cell products. P^igure 30 (Plate VI.) 

 represents the notochordal canal of an individual (a) which is one 

 of a pair of twins of Chelydra serpentina shown ip surface view in 

 Plate II. Fig. 5. The notochordal canal of this embryo has been 

 much retarded in its development, for reasons which will be stated 

 later. Even into this small lumen, which appears as though formed 

 in the main by a normal separation of cells, a few unstainable 

 shreds or fragments of cells (unfortunately not brought out by the 

 lithographer) project. Such an appearance along the walls of the 

 canal I have repeatedly observed. In a few instances, however, 

 there was no evidence of cells degenerating within the canal. 



We now come to the description of an invaginated cavity which 

 shows, I believe, an anomalous condition, at least one which has 

 not been described, to my knowledge, by writers on reptilian em- 

 bryology. The egg (Chrysemys picta) from which this shield was 

 obtained was found on June 16, 1893. The embryo was removed 

 three hours after the discovery of the Q%%- It was possible to 

 make use of the camera lucida to outline both dorsal and ventral 



