FATE OF BLASTOPORE, ETC., IN CHELOXIA. 59 



k is six sections behind /. There is a distinct median 

 groove, at the li])s of which the epihlast bends down to 

 become continuous with the mesoblast. The cjroove 

 begins in the second section after / and is ah-eady deep in 

 the fourth section. 



1 is five sections behind Jc. The «groove is shallower and not 

 so marked as in Iô. 



m is seven sections behind /. The yolk-plug appears in the 

 groove, first in the fourth section behind /. In this 

 section, it is very well marked and unmistakable. 



n is five sections behind m. The 3^olk-plug has gradually 

 united with the epiblast on both sides, but its place is 

 still marked by a slight arching upwards (indicated by 

 two stars in ^the figure) and by the flict of its being 

 united with the mesoblast below. After three or four 

 sections, the connection of the epiblast with the mesoblast 

 is lost, but the median elevation of the epiblast marking the 

 posterior prolongation of the yolk-plug is to he seen in this 

 scries for thirty seetions or more. 



Accompanying this series of transverse sections is given the 

 median longitudinal section of another embryo taken out from the 

 same deposit at the same time (Series X, long, VI. VIIL). It shows 

 nothing not seen in the transverse series, and only serves as a check 

 on it. The lines through this section show by their letters the levels 

 of the transverse sections indicated by the same letters. 



In this Clemmys embryo, the sections show that the tail-swelling 

 is not yet much raised. The primitive streak, Avhich in earlier stages 

 begjin from the posterior end of the medullary canal (Series III), has 

 now disappeared from over the " Endwulst," but behind this 

 mass, there are the streak and the groove, with the yolk-plug at the 

 posterior end. The posterior prolongation of the yolk-plug is very 

 long, but of this more anon. 



