Qg K. MITSUKURI; 



latter spreads itself over the ventral surface of the emhri/onic shield.'' (p. 251). 

 In the s;ime Contribution, I have given reasons for thinking that the 

 large yolk-mass of Chelonia (Ym. 2) ought to l)e regarded merely as a 

 ventral appendage of the archenteron, the cavity in which has arisen 

 because it is necessary to have the nutrient matter in the liquid form, 

 its presence being due, therefore, solely to physiological necessity and 

 having a comparatively insignificant morphological value.* 



3. 0. Hertwig (^95) has recently described some abnormal 

 embryos reared in salt solutions which remind us strongly of the 

 chelonian or selachian embryos. For instance, in his Figs. 15 and 16, 

 there is the tail projecting hook-like over the still exposed yolk-cells. 

 This is like Figs. 8 and 8rt of the present article, if we change the 

 relative sizes of the tail and the exposed yolk. While, therefore, there 

 can be no doubt that there is a general resem])lance in this respect 

 between the chelonian and selachian embryos on the one hand and the 

 amphibian embryos on the other, there is a serious doubt in my 

 own mind as to the homology of the yolk-plug in the Amphil)ia. It 

 seems to be tolerably certain from the works of many recent writers 

 that the edges of the circular yolk-plug in Amphibia, after a certain 

 stage, press towards the median line, and meet finally in the median 

 streak, leaving however an opening both at the anterior, and the 

 posterior, ends. The anterior opening l^ecomes the neurenteric canal 

 and the i^osterior the anus. If we look at Figs. 18, 19, and 23-26 of 

 RoBixsox and Asshetox ( 91) and read how the tail, with the exception 

 of the skin, is formed from the primitive streak, how the primitive 

 streak stretches backwards from the neurenteric canal, etc., I think, 

 w^e shall be oljliged to conclude that here we have the homologous 

 processes going on as in the posterior part of Chelonia and 



* I am well aware that these views are diametrically opposed to those recently advanced 

 by LwoFF ('94). Future investigations must decide which of us is correct. 



