FATE OF BLASTOPORE, ETC., IX CHELONIA. ji 



X is two sections behind ir. AH tlie parts enclosed within 

 the epiblast of the tail are now united, but a, |)ecu]iar 

 lobe-like [)rojection on the dorsal side indicates where the 

 medullary canal ought to be, and the two slits mark 

 the boundary between the mesoblast and hypoblast. 



y is two secti<His behind x. The mass ^vithin the tail woidd 

 be circular in outline but for a projection on the dorsal 

 side. 



Z is two sections behind //. The mass ^vi(hin now consists 

 entirely of an indifferent tissue, ^vitiKJut any sign of 

 differentiation into parts. 



This series sliows that the ])roctodœum is formed within tlie 

 limits of the ])rimitive streak, namely, in that part which runs from 

 the tip of the tail forward to the stalk of the embryo. 



The enteron Ijehind the proctodiiMun rapidly narrows and is soon 

 reduced to a string of cells (//) ; nevertheless it is continued to the 

 tail end, irilh tivo inU'riiqiliiHi^^ ]toirerer, in this embryo (at o, and l)ehind 

 ])). It would l)e retisonable to supjwse that at an earlier sta^e, the 

 hypoljlast string suffered no interruption. 



If we go gradually from : forwards, we see in what order the 

 different ])arts are di Itèrent ia(ed from the inditferent cell-mass within 

 the tail, as the latter elongates behind. 



C. SUIYIIYIARY OF THE FOREGOING FACTS. 



From tlie account given above, it is e^ddent that the mass which 

 I have called the yolk-})l ug undergoes a series of ver}' complex 

 changes. Neither the y\ew of Will *, that it disappears fVom the 

 surface when the blastopore lips close around it, nor that of Mehnekt, 

 that it finally fuses laterally with the hindmost part of the medul- 



* Wliich I accepted in Coutrib. IV. 



