14 Primitive Streak aitd NotocJiordal Canal in Chelonia. 



the ventral entodermal opening in the rabbit also. The dorsal 

 opening is so small, and the time of its existence so brief, that 

 Kolliker failed altogether to find it in the case of the rabbit, and 

 Lieberkiihn saw it but once in the mole. Hubrecht ('90, p. 509) 

 observed in the mole a thickening, which he has designated the 

 protochordal wedge. A slight canal enclosed within the ectoderm 

 appeared within this wedge. All efforts to find so distinct a canal 

 as Heape has recorded were in vain. Strahl ('86, p. 160, Taf. IV. 

 Fig. 7) figures and describes a distinct dorsal opening of the noto- 

 chordal canal in the cat. Heape ('83, p. 429) finds an ectodermal 

 involution at the anterior end of the streak of the mole in the 

 region in which the neurenteric canal had existed in younger 

 embryos, and considers the presence of this unobliterated rem- 

 nant of a canal evidence that the streak does not grow forward, but 

 must lengthen in a posterior direction. He ('86, p. 437) also dis- 

 cusses the presence of the neurenteric canal in younger embryos. 

 Bonnet ('84, '88, '89) has found the neurenteric canal in the 

 sheep. He states that the notochordal canal opens above and 

 below ('84, p. 217). The opening of the canal on the ventral side 

 is accomplished by a process of dehiscence ('88, p. 121). Dor- 

 sally the canal opens into the primitive groove of the streak ('88, 

 p. 122). Kolliker ('82) saw the ventral opening of the notochordal 

 canal, but later ('83, pp. 7 and 9) he stated that he found the ecto- 

 derm and entoderm united at the posterior end of the embryo 

 rabbit and an indication of a neurenteric canal. Van Beneden 

 (*86, p. 289) describes not only a dorsal and a ventral opening of 

 the notochordal canal in the bat, but also a mass of cells at the 

 posterior end of the canal which he homologizes with the Amphib- 

 ian yolk plug. He Q^6, p. 289) finds an opening of the canal at 

 the anterior end of the streak in both the rabbit and the bat. Graf 

 Spee ('88, p. 315) observed in the guinea-pig only a neurenteric 

 strand, but in the rabbit (p. 322) he found the neurenteric canal 

 to have a distinct lumen. Keibel (^2,d>, p. 410) described in 1888 

 a rudimentary neurenteric canal in the guinea-pig which did not 

 open dorsally, and in the following year he described a dorsal 

 opening which he observed in one instance ('89, p. 344). Robinson 

 ('92, Plate XXH. Fig. 13% Plate XXIV. Figs. 14* and 1$'^) figures 

 a neurenteric canal for the rat, and also (Plate XXVI. Fig. 17) 

 for the mouse. According to the observations of Fleischmann 

 i^d)"/, p. 12), the notochordal canal of the cat opens toward the 



