EARLY DEVELOPMENT OF JULUS TERRESTRIS: 465 
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 
Phriganids 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 
