202 CLARKE. [Vol. V. 



but if the part labelled "ai " in each of them be considered the 

 posterior end of the embryo with the neural ridges developing 

 in 3 and 4, then they all agree. It should be borne in mind 

 that ten of the figures on this PI. XI are monstrosities, as 

 stated in the foot-note at page 537. One of these figures of a 

 monstrous form is of special interest in comparison with the 

 alligator development : it is No. 8. This dorsal view is much 

 like Figs. 16 and 18 of the alligator, the two figures which 

 represent the peculiar method of formation of the cephalic end 

 of the neural tube as I have described it. This median growth 

 of the dorsal part of the head-fold posteriorly is very clearly 

 marked in Agassiz's figure, but it is carried much farther back- 

 ward than it extends in the alligator. Such a median fold of 

 the length there shown would be abnormal, too, for the alli- 

 gator ; but that there is anything abnormal in the two perfect 

 and symmetrical embryos shown in Figs. 16 and 18 of the 

 alligator there is no evidence for believing. 



In comparison with the structure given by Mitsukuri and 

 Ishikawa for the very early stages of Trionyx Japonicus there 

 is little to be said. There is no indication of anything unusual 

 in the method of formation of the cephalic end of the neural 

 folds, and in so far Trionyx is unlike the alligator. There are 

 no other marked differences to be discovered in the eiirht gene- 

 ral figures of Trionyx and those of similar stages of the alligator 

 or of Lacerta. The remaining figures of the turtles in Agassiz's 

 work agree very closely with those of the alligator up to the 

 late period when the shell and other special features of chelonian 

 structure begin to appear. 



Methods. 



I found it a difficult matter to remove the embryos during the 

 earlier stages, and after many trials met with much the best 

 success in the following way : The shell and its membrane are 

 carefully removed from nearly half the egg in an area extending 

 from end to end ; the egg is then poured out on to the left hand, 

 and the thick white at one end is cut off with scissors ; the egg 

 is then replaced in the shell with the germinal pole, still covered 

 with white, uppermost ; the white is now cut off from this end 

 as completely as possible ; the shell must then be placed in the 

 corner of a deep dish where it can stand on end ; a sharp-edged 



