FATE OF BLASTOPOEE, ETC., IN CHELOXIA. 21 



hinder end (the part dehneated dark in the figure), and the 

 nenrenteric canal occupies about the middle two-thirds of this part, 

 as can be readily determined by looking at the embryo by transmitted 

 light. The yolk-plug, which can be easily told by its characteristic 

 horse-shoe shaped outline, has therefore already considerably moved 

 itself back, although it is still superposed, in the dorsal view, on the 

 " Endwulst." This embryo is somewhat exceptional in that the 

 groove between the posterior end of the medullary canal and the yolk- 

 plug is ill defined, being shallower and wider than is usual, as a com- 

 parison with the succeeding figures will show. 



In Fig. 4, the backward progress of the yolk- plug has gone one 

 step further. As in Fig. 3, the posterior end of the medullary canal 

 is still left open, as shown by the dark streak. This opening is 

 somewhat irregular in shape, being wider in the middle and pro- 

 duced into narrower prolongations both in front and behind. The 

 neurenteric canal is at the posterior end of the middle wider part. 

 Now, the part shaded slightly lighter lying immediately around 

 this wider part shows approximately the extent of the medullary 

 canal as determined by transmitted light, so that all that part of 

 the dark streak behind the middle wider part must be regarded as 

 the groove stretching between the medullary canal and the yolk-plug. 

 In this specimen, tlie anterior half of this groove is wider and appears 

 as the direct posterior prolongation of that irregular opening which is 

 still left in the posterior end of the medullary canal. At about the 

 middle point of the groove, it becomes suddenly narrower and finer, 

 and reaches the anterior edge of the yolk-plug. The latter, horse-shoe 

 shaped as usual, appears now superposed on the edge of the 

 " Endwulst." 



In the embryo represented in its dorsal and ventral aspects in 

 Figs. D (PI. I.) and 5a (PI. II.), the amniotic fold has extended 



