EAELY DEVELOPMENT OP JULUS TBEEESTRIS. 235 



Julus terrestris inasmuch as the segments are repre- 

 sented by protoplasmic masses each of which is provided with 

 a nucleus. 



The formation of the blastoderm, as I have described it, 

 agrees in the main with that given by Metschnikoff for Stron- 

 gylosoma. According to this author, on the fifth day isolated 

 masses of cells make their appearance on the surface of the 

 ovum and spread themselves round it to form the blastoderm. 

 He was unable to trace the origin of these masses of cells. 

 What he saw was precisely what I have described in the earlier 

 part of this paper. 



The formation of the blastoderm in Julus is, then, such as is 

 generally found in tracheate development. 



The cells which at the conclusion of the blastoderm forma- 

 tion in Julus remain within the yolk, represent the endoderm, 

 and have apparently been overlooked by Metschnikoff. 



The mode of formation of the mesoderm almost exactly 

 resembles that described by Balfour (16) for Spiders. Accord- 

 ing, however, to this author the greater part of the cells of the 

 keel or ridge are derived from the ectoderm, whereas in Julus 

 the ectoderm furnishes the greater part of them. Balfour 

 states that the keel in Spiders is probably the homologue of 

 the mesoblastic groove of the insect blastoderm. Patten (12) 

 describes a median longitudinal furrow in the ventral plate of 

 Phryganids which gives rise to the mesoblast and to part of 

 the endoderm. 



In Peripatus (17) the mesoblast originates from the primitive 

 streak, i. e. from the indifferent tissue behind the blastopore, 

 which can be called neither ectoderm nor endoderm. I think 

 that all these structures are homologous. 



With regard to the cells which, as I have already mentioned, 

 are employed, neither in the formation of the keel nor at a 

 later period in the formation of the mesenteron, but remain in 

 the body cavity as mesoderm cells directly descended from endo- 

 derm — Balfour states that in Agelena, after the establishment 

 of the hypoblast the cells remaining in the yolk are not entirely 

 hypoblastic, since they continue for the greater part of the 



