ON THE DEVELOPMENT OF NEBALIA. 399 



blastoderm has not as yet completely surrounded the yolk. 

 Therefoi'e they must arise quite independently of the invagi- 

 nation. 



(2) From fig-. 6 it would seem that the mesoderm in this 

 region arose also from the blastoderm independently of the 

 invagination. This mesoderm Bergli calls metauauplial. The 

 mesoderm of the nauplius appendages he calls nauplial, and 

 says it is probably budded off from the ectoderm in situ. 

 This obliges him to admit of two origins for the mesoderm. 



(a) The invagination at the blastopore for the metauauplial 

 mesoderm. 



{})) Budding off from the blastoderm in situ for the 

 nauplial mesoderm. 



Now in Nebalia it seems to me that there are two possi- 

 bilities, either 



(1) All the mesoderm cells are budded off in situ from 

 the blastoderm, i. e. the nauplial mesoderm in the places 

 where the appendages are about to be folded off, and the 

 metauauplial mesoderm from the blastoderm in the region of 

 the blastopore at about the same time as the invagination 

 takes place but quite independently of it — or 



(2) All the mesoderm is budded off from the blastoderm in 

 the region of the blastopore at about the same time as the 

 invagination takes place (but quite independently of the 

 invagination), and the cells which will form the nauplial 

 mesoderm travel forwards. 



Either of these possibilities seems a little more probable 

 than the possibility that in the same animal part of the 

 mesoderm should arise as the result of an invagination and 

 part by division in situ. 



Butschinsky (1900) is of opinion that both endoderm and 

 mesoderm originate from a mass of cells to which in this stage 

 the blastoderm gives rise at the caudal thickening ; and this 

 is the only point of any importance in which our accounts of 

 the development differ. 



Putting aside the difference with regard to the origin of 

 the metauauplial mesoderm^ there is a striking similarity 



VOL. 50, PART 3. NEW SEKIES. 29 



