STUDIES IN MAMMALIAN EMBRYOLOGY. 541 



rendered extremely difficult. He has noticed a hypoblastic 

 thickening in this anterior region, but only holds it to be of 

 significance for the formation of the primitive " Rachenhaut " 

 (pharyngeal membrane), whereas a direct comparison with the 

 shrew's protochordal plate and with the chorda-entoblast 

 which Bonnet describes and figures in the sheep would perhaps 

 lead to difi'erent results. That Keibel feels all the importance 

 of the contradiction between his own views and those of 

 Bonnet (which are so fully supported by the facts observed 

 in the shrew) may be concluded from the following passage 

 which I translate from p. 345 of his article. He says, " I 

 will not here further refer to Bonnet's doctrine of the meso- 

 blast derived from the entoblast. Nothing like it is found 

 either in the rabbit or in the guinea-pig, and even Bonnet's 

 own figures have not convinced me. . . . Bonnet's observa- 

 tions are, at all events, not available in support of Rabl's or 

 van Beneden's hypothesis. If they were confirmed this would 

 mean a further difficult complication of our problem, and for 

 this reason I believe I may be relieved from further entering 

 upon Bonnet's data." 



If we refer to van Beneden's early article on the rabbit's 

 development {' Archives de Biologic,' vol. i, 1880), we find 

 that he confounds for the earlier stage (vi) epiblast and meso- 

 blast (pi. vi, fig. 2), regarding the trophoblast (Rauber's 

 Deckzellen) as the definite epiblast. This confusion has been 

 refuted by Lieberkiihn, and seems to have since been recog- 

 nised as such by the author, who on the same plate (figs. 

 11 — 13) gives correct interpretations of the three layers in a 

 later stage (ix), a stage of which he definitely affirms that 

 there was as yet no trace of Hensen's uode (i. e. of the gas- 

 trula ridge). These three figures should be somewhat more 

 carefully considered by us. They show that mesoblast is 

 present in the rabbit before there is any trace of the gastrula 

 ridge. This van Beneden emphatically states on p. 220, sub. 

 13. This mesoblast appears in crescent shape iu the posterior 

 region of the embryonic shield. Van Beneden does not men- 

 tion how he has made out that this was indeed the posterior 



