68 Journal op tiik Mitcheix Society [Septemher 



been mounted whole or sectioned. In CJinjsemys cinerea, the blasto- 

 pore is well within the germinal disc and is usually located asym- 

 metrically in reference to length, about one-third of the total length 

 distant from the posterior end. Figure 30 shows a typical case. Tn 

 some species the blastopore seems to be in such a position as to ex- 

 tend beyond the end of the disc, as figured by Mitsukuri (1894). Sev- 

 eral stages of the closing of the blastopore are shown in the mounted 

 specimens. The opening, which was originally round, comes to form 

 first an oval and then an almost straight slit which later becomes 

 slightly curved with the points of the crescent towards the anterior 

 part of the body. 



The cavity formed by the invagination at first pushes directly 

 downward and then suddenly bends cephalad, forming two layers 

 of ectodermal cells, the small space between which is probably the 

 segmentation cavity. The space continuous with the blastojiorie canal 

 is the arelienteron, the roof of which is formed by the invaginated 

 cells and the floor by the cells of the yolk endoderm. ]\Iitsukuri 

 (1894) was the first to identify this cavity in Trionij.r as the arch- 

 enteron. Older specimens were found in which the endoderm had 

 disappeared from the floor and the invagination cavity was continuous 

 with the space at the top of the yolk. Will (1893) and ^Mitsukuri 

 have had considerable argument as to the extent of this archenteronic 

 cavity before its entrance into the yolk cavity. The former held 

 that it was co-extensive with the disc while the latter maintained 

 that it was in no way approximately equal in size to the disc. Ob- 

 servations upon Chrysemys cinerea confirm the idea of Mitsukuri. 

 However, Will may also be correct for the form which he discussed, 

 since generic differences have been shown to exist. 



The primitive knot is on the posterior side of the blastopore. 

 When it is not covered with ectoderm it is considered a yolk plug. 

 Such an uncovered knob was found by jMitsukuri and Tshikawa in 

 18S6 in Trionyjr japonicus, which is one of the snapjiing turtles. They 

 at once formulated the idea of homology of the yolk ])lug in reptiles 

 and amphibians. Robinson and Asshenton (1891) disagree with the 

 homology- suggested by ^litsukuri and Ishikawa. Kupffer (1882) 

 did not find the yolk plug in r('])tilia. flitli/drd srrp( )ifi)i(i is also a 

 species of the "snapper," and shows this yolk i^lug as is seen in 

 figures 31. 32, 35, and 36. These drawings were made from mounts 

 alreadv in the T'niversitv of Wisconsin embrvological collections. 



