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E. W. MACBRIDE. 
gut, and he prudently withdraws his earlier statement 
that the left head cavity was of ectodermal origin. 
Reviewing the observations which have so far been sum- 
marised, we see that two questions stand out as pre-eminently 
calling for decision : first, is the roof of the archenteron, and 
consequently the mesodeian and notochord Avhich arise from 
it, ectodermal or endodermal ? And secondly, does the blasto- 
pore close by a seam-like concrescence proceeding from in 
front backwards or not ? A third question to be resolved is 
Avhether the comparison which I instituted in 1898 between 
Amphioxus and Bala nogloss us, so far as the formation 
of the mesoderm is concerned, can be sustained. 
Material and Methods. 
The material at my disposal consisted of a profusion of eggs 
and embryos covering all stages from the spherical blastula 
up to the period when the mouth, club-shaped gland, and one 
gill-slit have been formed. There were also a number of older 
larvae, but a considerable gap intervened between them and 
the larvie with one gill-slit, and in this paper I consider only 
the material which formed a series without any gaps. Next, 
as to preservation, the material was preserved partly in cor- 
rosive sublimate and acetic acid, partly in Kleinenberg’s 
picrosulphuric acid, partly in Fleming’s fluid, and partly in 
osmic acid. I had material of all stages preserved in cor- 
rosive sublimate and in osmic acid, and in the accuracy and 
fineness with which histological detail is preserved there is no 
other reagent which will stand comjDarison with osmic acid. 
This is, of course, a conclusion that is becoming more and 
more widely accepted, and some histologists go so far as to 
maintain that osmic acid is the only reagent which retains in 
dead protoplasm anything like the organisation it possessed 
in life. When, however, yolk is extremely abundant osmic 
acid makes the material so brittle that section cutting is 
almost impracticable ; and for this reason, in dealing with 
the stages of gastrulation I have used material preserved in 
