ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
17 
which is bounded anteriorly by the dorsal lip of the blastopore, is the 
most anterior portion of the blastopore. 
As to the fate of the streak, they report that the ventral moiety, soon 
after the perforation of the anus, ceases to be functional and splits up ; 
the dorsal half, on the other hand, becomes folded on itself, like and 
along with the neural plate ; it becomes separated from the skin and 
gives rise to the whole of the tail, with the exception of the greater part 
of the skin. 
During the completion of the archenteron and before the closure of 
the blastopore, the mesoblast in front of the latter begins to separate 
from the true hypoblast along the dorso-lateral aspects of the archen- 
teron by a process of delamination. Along the lines of attachment of 
the mesoblastic plates to the hypoblast, at the sides of the notochord, 
slight depressions may be noticed, which appear to indicate a continua- 
tion of the archenteric cavity into the mesoblast in the manner suggested 
by O. Hertwig. The appearances in the region of the primitive streak 
are such as to suggest the idea that the mesoblast is thus formed partly 
from the epiblast and partly from the hypoblast ; but there is no definite 
proof of this. 
The small archenteric diverticula into the mesoblast disappear after 
a time ; they do not, it would seem, communicate with the coelom, which 
is formed by a splitting of the mesoblast which first occurs laterally, 
and then extends dorsally and ventrally as in the higher Vertebrata. 
B. Histology. 
Structure of Protoplasm.* — Prof. O. Biitschli gives an interesting 
account of the various opinions which have been or are held in regard 
to the structure of protoplasm. The older observers, such as Dujardin 
and von Mohl, regarded it as a “ structureless ” jelly or slime ; it was 
called a fluid, a semi-solid, a fluid with unique properties, a fluid in a state 
of aggregation and so on. Briicke (1861), Cicnkowsky (1863), Yelten, 
Hanstein, began to distinguish in the protoplasm a relatively stable 
framework from a more fluid included substance. Frommann (1864-7) 
and Arnold (1865-7) emphasized this reticulate or fibrillar structure of 
the plasma. 
The most important opinions at present held in regard to the 
structure of protoplasm are as follows : — According to Frommann pro- 
toplasm consists of a relatively stable reticulate framework and a less 
dense, less refractive, less stable substance in the meshes. Many 
histologists of great authority accept this conclusion. According to 
others, such as Flemming, the darker and denser portion of the pro- 
toplasm is not a network so much as an irregular coil of fibrils. In 
contrast to both of the above conclusions, Berthold, Schwartz, Kolliker 
and others have returned to the old view of protoplasm, regarding it as 
a kind of emulsion, regarding reticulum and fibrils as artificial products. 
A fourth view is that of Altmann who regards the plasma as homo- 
geneous jelly with very numerous vital granules imbedded in it, in fact 
as a zooglcea of peculiar Bacteria. To Biitschli, finally, the reticulum is 
but the expression of the vacuolar or foamy nature of the protoplasm. 
Prof. Biitschli criticizes the various theories which regard proto- 
* Sep. Abd. Verb. Deutsch. Zool. Gesell., 1891, pp. 14-20. 
1892. 
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